Java Basics

1. Levels in Java

The source notes four levels at which Java members and code can be considered:

2. About main() Method

Every Java program execution starts with the main() method. To run a Java program, the source states that JRE must be installed in the system.

public static void main(String[] args) {
    // statements
}

At runtime, the JVM searches for the main method using the expected prototype:

public static void main(String args[])

Meaning of Each Part

3. main() Method Signature Forms

The order of the modifiers is not required to be fixed, and the String array can be declared in acceptable equivalent forms.

public static void main(String… args)
public static void main(String args[])
public static void main(String[] args[])
public static void main(String []args)
static public void main(String args[])

Summary from the Source

• The main() method is a static method.
• You can overload main() in Java.
• You cannot override main() in Java.
• The main method can be declared final, strictfp, or synchronized.
• You cannot call a non-static method directly from a static main method without an object.
• The JVM creates a main thread to execute the main method.

4. About System.out.println()

The source breaks System.out.println() into its Java components.

Field Summary

System.out.println(data);

5. Areas of Java Language

The source divides the Java runtime areas into two broad categories:

6. Java Variables

Variables are used to hold values that help achieve application requirements and functionality. When declaring a variable, its data type must be specified.

Three Types of Variables in Java

7. Local Variables

Variables declared inside a method, constructor or block are called local variables.

• They can be used only inside the method, constructor or block in which they are declared.
• They cannot be accessed directly from outside that method, constructor or block.
• Memory is allocated when the method or constructor starts and released when it completes.
• The source states that local variables are stored in stack memory.

Example

class Test {
    public static void main(String[] args) {
        int a = 10;   // local variable
        int b = 20;   // local variable

        System.out.println(a);
        System.out.println(b);
    }
}

8. Instance Variables

Variables declared inside a class and outside methods are called instance variables.

• They are visible in the methods and constructors of the particular class.
• Instance variables are also known as non-static fields.
• They have class-wide visibility within the object context.
• They are accessed using an object/reference variable.
• Memory is allocated when the object is created.
• The source states that instance variables are stored in heap memory.

Example

class Test {
    int a = 10;
    int b = 20;

    public static void main(String[] args) {
        Test t = new Test();

        System.out.println(t.a);
        System.out.println(t.b);

        t.m1();
    }

    void m1() {
        System.out.println(a);
        System.out.println(b);
    }
}

9. Static Variables

Variables declared inside a class and outside methods with the static modifier are called static variables.

• Static variables are visible to the methods and constructors of the class.
• Static members can be accessed using the class name.
• The source states that memory is allocated when the .class file is loaded and released when it is unloaded.
• The source describes static variables as being stored in non-heap memory.

Example

class Test {
    static int a = 1000;
    static int b = 2000;

    public static void main(String[] args) {
        System.out.println(a);
        System.out.println(b);

        Test t = new Test();
        t.m1();
    }

    void m1() {
        System.out.println(a);
        System.out.println(b);
    }
}

10. Accessing Static Variables

The source gives three ways to access static members inside static code:

1. Directly
2. Using the class name
3. Using a reference variable

Example

class Test {
    static int x = 100;

    public static void main(String[] args) {
        System.out.println(x);        // 1. directly
        System.out.println(Test.x);   // 2. using class name

        Test t = new Test();
        System.out.println(t.x);      // 3. using reference variable
    }
}

11. Variables vs Default Values

Case 1: Instance Variables

For instance variables, the JVM provides default values based on their data types.

class Test {
    int a;
    boolean b;

    public static void main(String[] args) {
        Test t = new Test();

        System.out.println(t.a);
        System.out.println(t.b);
    }
}

Case 2: Static Variables

For static variables, the JVM also provides default values based on their data types.

class Test {
    static int a;
    static float b;

    public static void main(String[] args) {
        System.out.println(Test.a);
        System.out.println(Test.b);
    }
}

Case 3: Local Variables

The JVM does not provide default values for local variables before use. A local variable must be initialized before it is read; otherwise the compiler reports an error such as variable a might not have been initialized.

class Test {
    public static void main(String[] args) {
        int a, b;

        System.out.println(a);  // compilation error
        System.out.println(b);  // compilation error
    }
}

12. Instance vs Static Variables

13. Class vs Object

14. Characteristics of Variables

15. java.util.Scanner — Dynamic Input

The source introduces the Scanner class for taking dynamic input from the keyboard.

• Scanner is present in the java.util package.
• The source notes that Scanner was introduced in Java 1.5.
• It is used to take dynamic input from the keyboard.

Create Scanner Object

Scanner s = new Scanner(System.in);

Common Scanner Methods

16. Scanner Program — Dynamic Employee Input

import java.util.*;

class Test {
    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);

        System.out.println("Enter emp hobbies");
        String ehobbies = s.nextLine();

        System.out.println("Enter emp no");
        int eno = s.nextInt();

        System.out.println("Enter emp name");
        String ename = s.next();

        System.out.println("Enter emp salary");
        float esal = s.nextFloat();

        System.out.println("***** emp details *****");
        System.out.println("emp no -------> " + eno);
        System.out.println("emp name -----> " + ename);
        System.out.println("emp sal ------> " + esal);
        System.out.println("emp hobbies --> " + ehobbies);

        s.close();
    }
}

17. Quick Revision