Week 4 [Fri, Aug 28th] - Topics

Casting is the action of converting from one type to another. You can use the (newType) syntax to cast a value to a type named newType.

When you cast a primitive value to another type, there may be a loss of precision.

double d = 5.3;
System.out.println("Before casting to an int: " + d);
int i = (int)d; // cast d to an int
System.out.println("After casting to an int: " + i);
d = (double)i; // cast i back to a double
System.out.println("After casting back a double: " + d);

Before casting to an int: 5.3
After casting to an int: 5
After casting back a double: 5.0

Downcasting is when you cast an object reference from a superclass to a subclass.

class Animal{
    void speak(){
        System.out.println("I'm an animal");
    }
}

class Cat extends Animal{
    @Override
    void speak() {
        System.out.println("I'm a Cat");
    }
}

class DomesticCat extends Cat{
    @Override
    void speak() {
        System.out.println("I'm a DomesticCat");
    }
}

public static void foo(Animal a){
    a.speak();
    Cat c = (Cat)a; // downcast a to a Cat
    c.speak();
    DomesticCat dc = (DomesticCat)a; // downcast a to a DomesticCat
    dc.speak();
}

Upcasting is when you cast an object reference from a subclass to a superclass. However, upcasting is done automatically by the compiler even if you do not specify it explicitly.

Cat c = new Cat();
Animal a1 = (Animal)c; //upcasting c to the Animal class
Animal a2 = c; //upcasting is implicit

Note that due to polymorphism, the behavior of the object will reflect the actual type of the object irrespective of the type of the variable holding a reference to it.

Animal a = new DomesticCat(); //implicit upcast
a.speak();
Cat c = (Cat)a; //downcast
c.speak();
DomesticCat dc = (DomesticCat)a; //downcast
dc.speak();

I'm a DomesticCat
I'm a DomesticCat
I'm a DomesticCat

Casting to an incompatible type can result in a ClassCastException at runtime.

Object o = new Animal();
Integer x = (Integer)o;

Exception in thread "main" java.lang.ClassCastException: misc.casting.Animal cannot be
cast to java.lang.Integer at misc.casting.CastingExamples.main(CastingExamples.java:14)

You can use the instanceof operator to check if a cast is safe to perform.

Cat c;
if (a instanceof Cat){
    c = (Cat)a;
}

You can declare a class as abstract when a class is merely a representation of commonalities among its subclasses in which case it does not make sense to instantiate objects of that class.

A class that has an abstract method becomes an abstract class because the class definition is incomplete (due to the missing method body) and it is not possible to create objects using an incomplete class definition.

In Java, an abstract method is declared with the keyword abstract and given without an implementation. If a class includes abstract methods, then the class itself must be declared abstract.

public abstract class Animal {

    protected String name;

    public Animal(String name){
        this.name = name;
    }
    public abstract String speak();
}

An abstract class is declared with the keyword abstract. Abstract classes can be used as reference type but cannot be instantiated.

public abstract class Account {

    int number;

    void close(){
        //...
    }
}

In Java, even a class that does not have any abstract methods can be declared as an abstract class.

When an abstract class is subclassed, the subclass should provide implementations for all of the abstract methods in its superclass or else the subclass must also be declared abstract.

public abstract class Feline extends Animal {
    public Feline(String name) {
        super(name);
    }

}

public class DomesticCat extends Feline {
    public DomesticCat(String name) {
        super(name);
    }

    @Override
    public String speak() {
        return "Meow";
    }
}

An interface is a behavior specification i.e. a collection of Just the method signature without any implementationmethod specifications. If a class implements all methods specified in an interfaceimplements the interface, it means the class is able to support the behaviors specified by the said interface.

There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a "contract" that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group's code is written. Generally speaking, interfaces are such contracts. _{--Oracle Docs on Java}

A class implementing an interface results in an is-a relationship, just like in class inheritance.

The text given in this section borrows some explanations and code examples from the -- Java Tutorial.

In Java, an interface is a reference type, similar to a class, mainly containing method signatures. Defining an interface is similar to creating a new class except it uses the keyword interface in place of class.

public interface DrivableVehicle {
    void turn(Direction direction);
    void changeLanes(Direction direction);
    void signalTurn(Direction direction, boolean signalOn);
    // more method signatures
}

Interfaces cannot be instantiated—they can only be implemented by classes. When an instantiable class implements an interface, indicated by the keyword implements, it provides a method body for each of the methods declared in the interface.

public class CarModelX implements DrivableVehicle {

    @Override
    public void turn(Direction direction) {
       // implementation
    }

    // implementation of other methods
}

An interface can be used as a type e.g., DrivableVechicle dv = new CarModelX();.

Interfaces can inherit from other interfaces using the extends keyword, similar to a class inheriting another.

public interface SelfDrivableVehicle extends DrivableVehicle {
   void goToAutoPilotMode();
}

Furthermore, Java allows multiple inheritance among interfaces. A Java interface can inherit multiple other interfaces. A Java class can implement multiple interfaces (and inherit from one class).

The design below is allowed by Java. In case you are not familiar with UML notation used: solid lines indicate normal inheritance; dashed lines indicate interface inheritance; the triangle points to the parent.

1. Staff interface inherits (note the solid lines) the interfaces TaxPayer and Citizen.
2. TA class implements both Student interface and the Staff interface.
3. Because of point 1 above, TA class has to implement all methods in the interfaces TaxPayer and Citizen.
4. Because of points 1,2,3, a TA is a Staff, is a TaxPayer and is a Citizen.

Interfaces can also contain constants and static methods.

public class Account{

  public static final double MAX_BALANCE = 1000000.0;

}

public interface DrivableVehicle {

    int MAX_SPEED = 150;

    static boolean isSpeedAllowed(int speed){
        return speed <= MAX_SPEED;
    }

    void turn(Direction direction);
    void changeLanes(Direction direction);
    void signalTurn(Direction direction, boolean signalOn);
    // more method signatures
}

Interfaces can contain default method implementations and nested types. They are not covered here.

You can organize your types (i.e., classes, interfaces, enumerations, etc.) into packages for easier management (among other benefits).

To create a package, you put a package statement at the very top of every source file in that package. The package statement must be the first line in the source file and there can be no more than one package statement in each source file. Furthermore, the package of a type should match the folder path of the source file. Similarly, the compiler will put the .class files in a folder structure that matches the package names.

package seedu.tojava.util;

public class Formatter {
    public static final String PREFIX = ">>";

    public static String format(String s){
        return PREFIX + s;
    }
}

Package names are written in all lower case (not camelCase), using the dot as a separator. Packages in the Java language itself begin with java. or javax. Companies use their reversed Internet domain name to begin their package names.

To use a public types contained in the packagepackage member from outside its package, you must do one of the following:

1. Use the type name prefixed by the package name e.g., java.io.IOExceptionfully qualified name to refer to the member
2. Import the package or the specific package member

package seedu.tojava;

import seedu.tojava.util.StringParser;
import seedu.tojava.frontend.*;

public class Main {

    public static void main(String[] args) {

        // Using the fully qualified name to access the Processor class
        String status = seedu.tojava.logic.Processor.getStatus();

        // Using the StringParser previously imported
        StringParser sp = new StringParser();

        // Using classes from the tojava.frontend package
        Ui ui = new Ui();
        Message m = new Message();

    }
}

Importing a package does not import its sub-packages, as packages do not behave as hierarchies despite appearances.

If you do not use a package statement, your type doesn't have a package -- a practice not recommended (except for small code examples) as it is not possible for a type in a package to import a type that is not in a package.

Optionally, a static import can be used to import static members of a type so that the imported members can be used without specifying the type name.

import static seedu.tojava.util.Formatter.PREFIX;
import static seedu.tojava.util.Formatter.format;

public class Main {

    public static void main(String[] args) {

        String formatted = format("Hello");
        boolean isFormatted = formatted.startsWith(PREFIX);
        System.out.println(formatted);
    }
}

package seedu.tojava.util;

public class Formatter {
    public static final String PREFIX = ">>";

    public static String format(String s){
        return PREFIX + s;
    }
}

When using the commandline to compile/run Java, you should take the package into account.

Access level modifiers determine whether other classes can use a particular field or invoke a particular method.

There are two levels of access control:

1. At the class level:

   • public: the class is visible to all classes everywhere
   • no modifier (the default, also known as package-private): it is visible only within its own package
     
     

2. At the member level:

   • public or no modifier (package-private): same meaning as when used with top-level classes
   • private: the member can only be accessed in its own class
   • protected: the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package

The following table shows the access to members permitted by each modifier.

Access levels affect you in two ways:

1. When you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use.
2. When you write a class, you need to decide what access level every member variable and every method in your class should have.

Well-written applications include error-handling code that allows them to recover gracefully from unexpected errors. When an error occurs, the application may need to request user intervention, or it may be able to recover on its own. In extreme cases, the application may log the user off or shut down the system. -- _Microsoft

Exceptions are used to deal with 'unusual' but not entirely unexpected situations that the program might encounter at runtime.

Most languages allow code that encountered an "exceptional" situation to encapsulate details of the situation in an Exception object and throw/raise that object so that another piece of code can catch it and deal with it. This is especially useful when the code that encountered the unusual situation does not know how to deal with it.

The extract below from the -- Java Tutorial (with slight adaptations) explains how exceptions are typically handled.

Advantages of exception handling in this way:

• The ability to propagate error information through the call stack.
• The separation of code that deals with 'unusual' situations from the code that does the 'usual' work.

Given below is an extract from the -- Java Tutorial, with some adaptations.

The content below uses extracts from the -- Java Tutorial, with some adaptations.

A program can catch exceptions by using a combination of the try, catch blocks.

• The try block identifies a block of code in which an exception can occur.
• The catch block identifies a block of code, known as an exception handler, that can handle a particular type of exception.

public void writeList() {
    print("starting method");
    try {
        print("starting process");
        process();
        print("finishing process");

    } catch (IndexOutOfBoundsException e) {
        print("caught IOOBE");

    } catch (IOException e) {
        print("caught IOE");

    }
    print("finishing method");
}

You can use a finally block to specify code that is guaranteed to execute with or without the exception. This is the right place to close files, recover resources, and otherwise clean up after the code enclosed in the try block.

The writeList() method below has a finally block:

public void writeList() {
    print("starting method");
    try {
        print("starting process");
        process();
        print("finishing process");

    } catch (IndexOutOfBoundsException e) {
        print("caught IOOBE");

    } catch (IOException e) {
        print("caught IOE");

    } finally {
        // clean up
        print("cleaning up");
    }
    print("finishing method");
}

Some possible outputs:

• The try statement should contain at least one catch block or a finally block and may have multiple catch blocks.

• The class of the exception object indicates the type of exception thrown. The exception object can contain further information about the error, including an error message.

You can use the throw statement to throw an exception. The throw statement requires a Throwable objects are instances of any subclass of the Throwable class.throwable object as the argument.

if (size == 0) {
    throw new EmptyStackException();
}

In Java, Checked exceptions are subject to the Catch or Specify Requirement: code that might throw checked exceptions must be enclosed by either of the following:

• A try statement that catches the exception. The try must provide a handler for the exception.
• A method that specifies that it can throw the exception. The method must provide a throws clause that lists the exception.

Unchecked exceptions are not required to follow to the Catch or Specify Requirement but you can apply the requirement to them too.

public void writeList() throws IOException, IndexOutOfBoundsException {
    print("starting method");
    process();
    print("finishing method");
}

Some possible outputs:

Java comes with a collection of built-in exception classes that you can use. When they are not enough, it is possible to create your own exception classes.

In general, use exceptions only for 'unusual' conditions. Use normal return statements to pass control to the caller for conditions that are 'normal'.

Proper naming improves the readability of code. It also reduces bugs caused by ambiguities regarding the intent of a variable or a method.

There are only two hard things in Computer Science: cache invalidation and naming things. _{-- Phil Karlton}

Every system is built from a domain-specific language designed by the programmers to describe that system. Functions are the verbs of that language, and classes are the nouns.
_{-- Robert C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship}

Use nouns for classes/variables and verbs for methods/functions.

Distinguish clearly between single-valued and multi-valued variables.

Person student;
ArrayList<Person> students;

name = 'Jim'
names = ['Jim', 'Alice']

Use correct spelling in names. Avoid 'texting-style' spelling. Avoid foreign language words, slang, and names that are only meaningful within specific contexts/times e.g. terms from private jokes, a TV show currently popular in your country.

A name is not just for differentiation; it should explain the named entity to the reader accurately and at a sufficient level of detail.

If a name has multiple words, they should be in a sensible order.

Imagine going to the doctor's and saying "My eye1 is swollen"! Don’t use numbers or case to distinguish names.

While it is preferable not to have lengthy names, names that are 'too short' are even worse. If you must abbreviate or use acronyms, do it consistently. Explain their full meaning at an obvious location.

Related things should be named similarly, while unrelated things should NOT.

Avoid misleading or ambiguous names (e.g. those with multiple meanings), similar sounding names, hard-to-pronounce ones (e.g. avoid ambiguities like "is that a lowercase L, capital I or number 1?", or "is that number 0 or letter O?"), almost similar names.

Be wary when a method is longer than the computer screen, and take corrective action when it goes beyond 30 LOC (lines of code). The bigger the haystack, the harder it is to find a needle.

If you need more than 3 levels of indentation, you're screwed anyway, and should fix your program. _{--Linux 1.3.53 Coding Style}

In particular, avoid arrowhead style code.

int subsidy() {
    int subsidy;
    if (!age) {
        if (!sub) {
            if (!notFullTime) {
                subsidy = 500;
            } else {
                subsidy = 250;
            }
        } else {
            subsidy = 250;
        }
    } else {
        subsidy = -1;
    }
    return subsidy;
}

int calculateSubsidy() {
    int subsidy;
    if (isSenior) {
        subsidy = REJECT_SENIOR;
    } else if (isAlreadySubsidized) {
        subsidy = SUBSIDIZED_SUBSIDY;
    } else if (isPartTime) {
        subsidy = FULLTIME_SUBSIDY * RATIO;
    } else {
        subsidy = FULLTIME_SUBSIDY;
    }
    return subsidy;
}

def calculate_subs():
    if not age:
        if not sub:
            if not not_fulltime:
                subsidy = 500
            else:
                subsidy = 250
        else:
            subsidy = 250
    else:
        subsidy = -1
    return subsidy

def calculate_subsidy():
    if is_senior:
        return REJECT_SENIOR
    elif is_already_subsidized:
        return SUBSIDIZED_SUBSIDY
    elif is_parttime:
        return FULLTIME_SUBSIDY * RATIO
    else:
        return FULLTIME_SUBSIDY

Avoid complicated expressions, especially those having many negations and nested parentheses. If you must evaluate complicated expressions, have it done in steps (i.e. calculate some intermediate values first and use them to calculate the final value).

return ((length < MAX_LENGTH) || (previousSize != length)) && (typeCode == URGENT);

boolean isWithinSizeLimit = length < MAX_LENGTH;
boolean isSameSize = previousSize != length;
boolean isValidCode = isWithinSizeLimit || isSameSize;

boolean isUrgent = typeCode == URGENT;

return isValidCode && isUrgent;

return ((length < MAX_LENGTH) or (previous_size != length)) and (type_code == URGENT)

is_within_size_limit = length < MAX_LENGTH
is_same_size = previous_size != length
is_valid_code = is_within_size_limit or is_same_size

is_urgent = type_code == URGENT

return is_valid_code and is_urgent

The competent programmer is fully aware of the strictly limited size of his own skull; therefore he approaches the programming task in full humility, and among other things he avoids clever tricks like the plague. _{-- Edsger Dijkstra}

When the code has a number that does not explain the meaning of the number, it is called a "magic number" (as in "the number appears as if by magic"). Using a e.g., PInamed constant makes the code easier to understand because the name tells us more about the meaning of the number.

return 3.14236;
...
return 9;

static final double PI = 3.14236;
static final int MAX_SIZE = 10;
...
return PI;
...
return MAX_SIZE - 1;

return 3.14236
...
return 9

PI = 3.14236
MAX_SIZE = 10
...
return PI
...
return MAX_SIZE - 1

Similarly, you can have ‘magic’ values of other data types.

return "Error 1432"; // A magic string!

return "Error 1432" # A magic string!

In general, try to avoid any magic literals.

Make the code as explicit as possible, even if the language syntax allows them to be implicit. Here are some examples:

• [Java] Use explicit type conversion instead of implicit type conversion.
• [Java, Python] Use parentheses/braces to show groupings even when they can be skipped.
• [Java, Python] Use enumerations when a certain variable can take only a small number of finite values. For example, instead of declaring the variable 'state' as an integer and using values 0, 1, 2 to denote the states 'starting', 'enabled', and 'disabled' respectively, declare 'state' as type SystemState and define an enumeration SystemState that has values 'STARTING', 'ENABLED', and 'DISABLED'.

Lay out the code so that it adheres to the logical structure. The code should read like a story. Just like how you use section breaks, chapters and paragraphs to organize a story, use classes, methods, indentation and line spacing in your code to group related segments of the code. For example, you can use blank lines to group related statements together.

Sometimes, the correctness of your code does not depend on the order in which you perform certain intermediary steps. Nevertheless, this order may affect the clarity of the story you are trying to tell. Choose the order that makes the story most readable.

Avoid things that would make the reader go ‘huh?’, such as,

• unused parameters in the method signature
• similar things that look different
• different things that look similar
• multiple statements in the same line
• data flow anomalies such as, pre-assigning values to variables and modifying it without any use of the pre-assigned value

As the old adage goes, "keep it simple, stupid” (KISS). Do not try to write ‘clever’ code. For example, do not dismiss the brute-force yet simple solution in favor of a complicated one because of some ‘supposed benefits’ such as 'better reusability' unless you have a strong justification.

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. _{-- Brian W. Kernighan}

Programs must be written for people to read, and only incidentally for machines to execute. _{-- Abelson and Sussman}

Optimizing code prematurely has several drawbacks:

• You may not know which parts are the real performance bottlenecks. This is especially the case when the code undergoes transformations (e.g. compiling, minifying, transpiling, etc.) before it becomes an executable. Ideally, you should use a profiler tool to identify the actual bottlenecks of the code first, and optimize only those parts.
• Optimizing can complicate the code, affecting correctness and understandability.
• Hand-optimized code can be harder for the compiler to optimize (the simpler the code, the easier it is for the compiler to optimize). In many cases, a compiler can do a better job of optimizing the runtime code if you don't get in the way by trying to hand-optimize the source code.

A popular saying in the industry is make it work, make it right, make it fast which means in most cases, getting the code to perform correctly should take priority over optimizing it. If the code doesn't work correctly, it has no value no matter how fast/efficient it is.

Premature optimization is the root of all evil in programming. _{-- Donald Knuth}

Note that there are cases where optimizing takes priority over other things e.g. when writing code for resource-constrained environments. This guideline is simply a caution that you should optimize only when it is really needed.

Avoid varying the level of abstraction within a code fragment. Note: The Productive Programmer (by Neal Ford) calls this the SLAP principle i.e. Single Level of Abstraction Per method.

readData();
salary = basic * rise + 1000;
tax = (taxable ? salary * 0.07 : 0);
displayResult();

readData();
processData();
displayResult();

The happy path (i.e. the execution path taken when everything goes well) should be clear and prominent in your code. Restructure the code to make the happy path unindented as much as possible. It is the ‘unusual’ cases that should be indented. Someone reading the code should not get distracted by alternative paths taken when error conditions happen. One technique that could help in this regard is the use of guard clauses.

if (!isUnusualCase) {  //detecting an unusual condition
    if (!isErrorCase) {
        start();    //main path
        process();
        cleanup();
        exit();
    } else {
        handleError();
    }
} else {
    handleUnusualCase(); //handling that unusual condition
}

if (isUnusualCase) { //Guard Clause
    handleUnusualCase();
    return;
}

if (isErrorCase) { //Guard Clause
    handleError();
    return;
}

start();
process();
cleanup();
exit();

The first version of the code you write may not be of production quality. It is OK to first concentrate on making the code work, rather than worry over the quality of the code, as long as you improve the quality later. This process of improving a program's internal structure in small steps without modifying its external behavior is called refactoring.

• Refactoring is not rewriting: Discarding poorly-written code entirely and re-writing it from scratch is not refactoring because refactoring needs to be done in small steps.
• Refactoring is not bug fixing: By definition, refactoring is different from bug fixing or any other modifications that alter the external behavior (e.g. adding a feature) of the component in concern.

Given below are two common refactorings (more).

if (isSpecialDeal()) {
    total = price * 0.95;
    send();
} else {
    total = price * 0.98;
    send();
}

if (isSpecialDeal()) {
    total = price * 0.95;
} else {
    total = price * 0.98;
}
send();

if is_special_deal:
    total = price * 0.95
    send()
else:
    total = price * 0.98
    send()

if is_special_deal:
    total = price * 0.95
else:
    total = price * 0.98
    
send()

void printOwing() {
    printBanner();

    // print details
    System.out.println("name:	" + name);
    System.out.println("amount	" + getOutstanding());
}

void printOwing() {
    printBanner();
    printDetails(getOutstanding());
}

void printDetails(double outstanding) {
    System.out.println("name:	" + name);
    System.out.println("amount	" + outstanding);
}

def print_owing():
    print_banner()

    # print details
    print("name:	" + name)
    print("amount	" + get_outstanding())

def print_owing():
    print_banner()
    print_details(get_outstanding())

def print_details(amount):
    print("name:	" + name)
    print("amount	" + amount)

Some IDEs have builtin support for basic refactorings such as automatically renaming a variable/method/class in all places it has been used.

Refactoring, even if done with the aid of an IDE, may still result in regressions. Therefore, each small refactoring should be followed by regression testing.

• Introduction to Refactoring (in IntelliJ IDEA): An article on the refactorings available in IntelliJ IDEA.

Given below are some more commonly used refactorings. A more comprehensive list is available at refactoring-catalog.

1. Consolidate Conditional Expression
2. Decompose Conditional
3. Inline Method
4. Remove Double Negative
5. Replace Magic Literal
6. Replace Nested Conditional with Guard Clauses
7. Replace Parameter with Explicit Methods
8. Reverse Conditional
9. Split Loop
10. Split Temporary Variable

You know that it is important to refactor frequently so as to avoid the accumulation of ‘messy’ code which might get out of control. But how much refactoring is too much refactoring? It is too much refactoring when the benefits no longer justify the cost. The costs and the benefits depend on the context. That is why some refactorings are ‘opposites’ of each other (e.g. extract method vs inline method).

The PR review stage is a dialog between the PR author and members of the repo that received the PR, in order to refine and eventually merge the PR.

Given below are some steps you can follow when reviewing a PR.

1. Locate the PR:

1. Go to the GitHub page of the repo.
2. Click on the Pull requests tab.
3. Click on the PR you want to review.

2. Read the PR description. It might contain information relevant to reviewing the PR.

3. Click on the Files changed tab to see the diff view.

4. Add review comments:

1. Hover over the line you want to comment on and click on the icon that appears on the left margin. That should create a text box for you to enter your comment.
   To mark multiple lines, click-and-drag the icon.
2. Enter your comment.
   This page @SE-EDU/guides has some best practices PR reviewers can follow.
3. After typing in the comment, click on the Start a review button (not the Add single comment button. This way, your comment is saved but not visible to others yet. It will be visible to others only when you have finished the entire review.
   

4. Repeat the above steps to add more comments.

5. Submit the review:

1. When there are no more comments to add, click on the Review changes button (on the top right of the diff page).
2. Type in an overall comment about the PR, if any. e.g.,

   Overall, I found your code easy to read for the most part except a few places
   where the nesting was too deep. I noted a few minor coding standard violations
   too. Some of the classes are getting quite long. Consider splitting into smaller
   classes if that makes sense.
   

   LGTM is often used in such overall comments, to indicate Looks good to merge.
   nit is another such term, used to indicate minor flaws e.g., LGTM, almost. Just a few nits to fix..
3. Choose Approve, Comment, or Request changes option as appropriate and click on the Submit review button.

Branching is the process of evolving multiple versions of the software in parallel. For example, one team member can create a new branch and add an experimental feature to it while the rest of the team keeps working on another branch. Branches can be given names e.g. master, release, dev.

A branch can be merged into another branch. Merging usually results in a new commit that represents the changes done in the branch being merged.

Merge conflicts happen when you try to merge two branches that had changed the same part of the code and the RCS cannot decide which changes to keep. In those cases, you have to ‘resolve’ the conflicts manually.

Git supports branching, which allows you to do multiple parallel changes to the content of a repository.

A Git branch is simply a named label pointing to a commit. The HEAD label indicates which branch you are on. Git creates a branch named master by default. When you add a commit, it goes into the branch you are currently on, and the branch label (together with the HEAD label) moves to the new commit.

Given below is an illustration of how branch labels move as branches evolve.

1. There is only one branch (i.e., master) and there is only one commit on it.
2. A new commit has been added. The master and the HEAD labels have moved to the new commit.
3. A new branch fix1 has been added. The repo has switched to the new branch too (hence, the HEAD label is attached to the fix1 branch).
4. A new commit (c) has been added. The current branch label fix1 moves to the new commit, together with the HEAD label.
5. The repo has switched back to the master branch.

6. A new commit (d) has been added. The master label has moved to that commit.
7. The repo has switched back to the fix1 branch and added a new commit (e) to it.
8. The repo has switched to the master branch and the fix1 branch has been merged into the master branch, creating a merge commit f. The repo is currently on the master branch.

Follow the steps below to learn how to work with branches. You can use any repo you have on your computer (e.g. a clone of the samplerepo-things) for this.

0. Observe that you are normally in the branch called master.

git status

on branch master

1. Start a branch named feature1 and switch to the new branch.

Click on the Branch button on the main menu. In the next dialog, enter the branch name and click Create Branch.

Note how the feature1 is indicated as the current branch.

You can use the branch command to create a new branch and the checkout command to switch to a specific branch.

git branch feature1
git checkout feature1

One-step shortcut to create a branch and switch to it at the same time:

git checkout –b feature1

2. Create some commits in the new branch. Just commit as per normal. Commits you add while on a certain branch will become part of that branch.
Note how the master label and the HEAD label moves to the new commit (The HEAD label of the local repo is represented as in SourceTree).

3. Switch to the master branch. Note how the changes you did in the feature1 branch are no longer in the working directory.

Double-click the master branch.

git checkout master

4. Add a commit to the master branch. Let’s imagine it’s a bug fix.
To keep things simple for the time being, this commit should not involve the same content that you changed in the feature1 branch. To be on the safe side, this commit can change an entirely different file.

5. Switch back to the feature1 branch (similar to step 3).

6. Merge the master branch to the feature1 branch, giving an end-result like the following. Also note how Git has created a merge commit.

git merge master

The objective of that merge was to sync the feature1 branch with the master branch. Observe how the changes you did in the master branch (i.e. the imaginary bug fix) is now available even when you are in the feature1 branch.

7. Add another commit to the feature1 branch.

8. Switch to the master branch and add one more commit.

9. Merge feature1 to the master branch, giving and end-result like this:

git merge feature1

10. Create a new branch called add-countries, switch to it, and add some commits to it (similar to steps 1-2 above). You should have something like this now:

11. Go back to the master branch and merge the add-countries branch onto the master branch (similar to steps 8-9 above). While you might expect to see something like the following,

... you are likely to see something like this instead:

That is because Git does a fast forward merge if possible. Seeing that the master branch has not changed since you started the add-countries branch, Git has decided it is simpler to just put the commits of the add-countries branch in front of the master branch, without going into the trouble of creating an extra merge commit.

It is possible to force Git to create a merge commit even if fast forwarding is possible.

Tick the box shown below when you merge a branch:

Use the --no-ff switch (short for no fast forward):

git merge --no-ff add-countries

Pushing a branch to a remote repo

Here's how to push a branch to a remote repo:

Here's how to push a branch named add-intro to your own fork of a repo named samplerepo-pr-practice:

Normally: git push {remote repository} {branch}. Examples:

• git push origin master pushes the master branch to the repo named origin (i.e., the repo you cloned from)
• git push upstream-repo add-intro pushes the add-intro branch to the repo named upstream-repo

If pushing a branch you created locally to the remote for the first time, add the -u flag to get the local branch to track the new upstream branch:
e.g., git push -u origin add-intro

See git-scm.com/docs/git-push for details of the push command.

Merge conflicts happen when you try to combine two incompatible versions (e.g., merging a branch to another but each branch changed the same part of the code in a different way).

Here are the steps to simulate a merge conflict and use it to learn how to resolve merge conflicts.

0. Create an empty repo or clone an existing repo, to be used for this activity.

1. Start a branch named fix1 in the repo. Create a commit that adds a line with some text to one of the files.

2. Switch back to master branch. Create a commit with a conflicting change i.e. it adds a line with some different text in the exact location the previous line was added.

3. Try to merge the fix1 branch onto the master branch. Git will pause mid-way during the merge and report a merge conflict. If you open the conflicted file, you will see something like this:

COLORS
------
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
white

4. Observe how the conflicted part is marked between a line starting with <<<<<< and a line starting with >>>>>>, separated by another line starting with =======.

Highlighted below is the conflicting part that is coming from the master branch:

blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red

This is the conflicting part that is coming from the fix1 branch:

blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red

5. Resolve the conflict by editing the file. Let us assume you want to keep both lines in the merged version. You can modify the file to be like this:

COLORS
------
blue
black
green
red
white

6. Stage the changes, and commit.