Java Building Blocks

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title: "Java Building Blocks" date: "2024-10-06" excerpt: "A practical summary of Java fundamentals: entry points, imports, compilation, classpath, literals, wrappers, text blocks, and garbage collection."

tags: "java,language-basics,certification" source: "https://shrikant-havale.in/2024/10/06/java-building-blocks/"

This article is a compact, exam-focused set of Java fundamentals and command-line basics.

Program Entry Point

• The main method accepts multiple valid parameter forms:
  • String[] args
  • String args[]
  • String... args
• final modifiers are optional in main signatures.

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

Launching Single-File Programs

• Standard compile/run flow:

javac Zoo.java
java Zoo

• Single-file launch shortcut:

java Zoo.java one two

Imports and Packages

• java.lang is auto-imported.
• Wildcards only apply at one package level.
• Class imports take precedence over wildcard imports.
• Classes in the same package do not need explicit imports.

import java.util.Random;
Random r = new Random();
int n = r.nextInt(10);

javac and java Commands

Compile

javac packagea/ClassA.java packageb/ClassB.java
javac -d classes packagea/ClassA.java packageb/ClassB.java

Classpath

javac -cp "C:\\temp\\directoryWithJars\\*" myPackage/MyClass.java
java -cp classes packageb.ClassB

• On Windows, classpath entries are separated by ;.
• On macOS/Linux, classpath entries are separated by :.

jar Command Basics

jar -cvf test.jar .
jar --create --verbose --file test.jar .
jar -cvf test.jar -C dir .

Initialization Order

• Fields and instance initializer blocks run in declaration order.
• Constructor runs after fields/initializers.

Numeric Literals and Types

• float literals require f/F.
• long literals often require L.
• Supports octal (017), hex (0xFF), binary (0b10).
• Numeric underscores are allowed with placement rules.

Wrapper Classes

int primitive = Integer.parseInt("123");
Integer wrapper = Integer.valueOf("123");
Double apple = Double.valueOf("200.99");

Useful conversions include intValue(), longValue(), doubleValue(), etc.

Text Blocks

• Require a line break after opening """.
• Incidental whitespace is trimmed relative to left margin rules.

Identifiers and var

• Identifiers cannot start with numbers.
• _ alone is not a valid identifier.
• Reserved words and literals (true, false, null) cannot be used as variable names.
• var is for local variable inference only.

Garbage Collection Notes

• System.gc() is only a request.
• Objects become GC-eligible when no accessible references remain.

public class Bear {
    private Bear pandaBear;

    private void roar(Bear b) {
        System.out.println("Roar!");
        pandaBear = b;
    }

    public static void main(String[] args) {
        Bear brownBear = new Bear();
        Bear polarBear = new Bear();
        brownBear.roar(polarBear);
        polarBear = null;
        brownBear = null;
        System.gc();
    }
}

Source

Original post: https://shrikant-havale.in/2024/10/06/java-building-blocks/