Java Notes

Source: Dev.to

Running Java from Terminal

javac App.java && java App

javac compiles the Java source file (App.java) into bytecode (App.class).
The && operator runs the second command only if the first succeeds, and java executes the compiled bytecode.

Hello World Example

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

Variables

Variables act as containers for values of various data types.

public class Main {
    public static void main(String[] args) {
        int age = 10;
        String name = "Davet";
        boolean isCorrect = true;
        float height = 6.4F;
        double weight = 6.0;
        char gender = 'M';
        byte a = 1;
        long number = 2334323423432423L;
    }
}

String Manipulation Methods

public class Main {
    public static void main(String[] args) {
        String name = "Davet";
        name.toLowerCase();          // "davet"
        name.toUpperCase();          // "DAVET"
        name.trim();                 // removes leading/trailing spaces
        name.replace("a", "b");      // replaces characters
    }
}

Enum

An enum defines a fixed set of constant values.

public class Main {
    public static void main(String[] args) {
        GRADE fail = GRADE.F;
        System.out.println(fail);
    }

    enum GRADE {
        A, B, C, D, E, F
    }
}

While Loop

The while loop repeatedly executes a block while a condition remains true.

public class Main {
    public static void main(String[] args) {
        boolean isCorrect = true;
        int count = 0;
        while (count < 10) {
            System.out.println("The answer you picked is correct");
            count++;
        }
    }
}

Functions (Methods)

A method groups code that performs a specific task and must be invoked to run.

public class Main {
    public static void main(String[] args) {
        introduce();
    }

    public static void introduce() {
        String name = "David";
        String jobDescription = "Backend Developer";
        System.out.printf("I am %s, a %s", name, jobDescription);
    }
}

Atomicity

Atomicity ensures that a series of operations either all succeed or none are applied.

import java.util.concurrent.AtomicInteger;

public class Main {
    static int count = 1;

    synchronized static void increment() {
        count++;
    }

    public static void main(String[] args) {
        increment();
        System.out.println(count); // prints 2

        // Using AtomicInteger
        AtomicInteger atomicInt = new AtomicInteger(1); // initialise with 1
        System.out.println(atomicInt.getAndIncrement()); // prints 1
        System.out.println(atomicInt.incrementAndGet()); // prints 3
        System.out.println(atomicInt.get()); // prints 3
    }
}