イニシャライザー

class User {
	private String name;
	private static int count;

	static {
		User.count = 0;
		System.out.println("Static initializer");
	}

	public User(String name){
		this.name = name;
		User.count++;
		System.out.println("Constructor initializer");
	}

	{
		System.out.println("Instance initializer");
	}

	public static void getInfo(){
		System.out.println("# of instances: " + User.count);
	}
}

public class MyApp {

	public static void main(String[] args){
		User.getInfo();
		User tom = new User("Tom");
		User.getInfo();
		User john = new User("John");
		User.getInfo();
	}
}

[vagrant@localhost java]$ javac MyApp.java
[vagrant@localhost java]$ java MyApp
Static initializer
# of instances: 0
Instance initializer
Constructor initializer
# of instances: 1
Instance initializer
Constructor initializer
# of instances: 2

final は継承できない。

final class User {
	protected String name;
	private static final double VERSION = 1.1;

	static {
		this.name = name;
		User.VERSION = 1.2;
	}

	public final void sayHi{
		System.out.println("Hi! " + this.name);
	}

	public static void getInfo(){
		System.out.println("# of instances: " + User.count);
	}
}

抽象クラス

// 抽象クラス -> 具象クラス

abstract class User {
	public abstract void sayHi(); // 抽象メソッド
}

class JapaneseUser extends User {
	@Override
	public void sayHi(){
		System.out.println("こんにちは！");
	}
}

class AmericanUser extends User {
	@Override
	public void sayHi(){
		System.out.println("Hi");
	}
}

public class MyApp {

	public static void main(String[] args){
		AmericanUser tom = new AmericanUser();
		JapaneseUser aki = new JapaneseUser();
		tom.sayHi();
		aki.sayHi();
	}
}

[vagrant@localhost java]$ javac MyApp.java
[vagrant@localhost java]$ java MyApp
Hi
こんにちは！

interface

interface Printable{
	// 定数
	double VERSION = 1.2;
	// 抽象メソッド
	void print();
	// default メソッド
	public default void getInfo(){
		System.out.println("I/F ver. " + Printable.VERSION);
	} 
	// static メソッド

} 

class User implements Printable {
	@Override
	public void print(){
		System.out.println("Now printing user profile...");
	}
}

public class MyApp {

	public static void main(String[] args){
		User tom = new User();
		tom.print();
		tom.getInfo();
	}
}

[vagrant@localhost java]$ javac MyApp.java
[vagrant@localhost java]$ java MyApp
Now printing user profile…
I/F ver. 1.2

列挙型

enum Result {
	SUCCESS,
	ERROR,

}


public class MyApp {

	public static void main(String[] args){
		Result res;

		res = Result.ERROR;

		switch (res){
			case SUCCESS:
				System.out.println("OK!");
				System.out.println(res.ordinal());
			break;
			case ERROR:
				System.out.println("NG!");
				System.out.println(res.ordinal());
				break;
		}

		
	}
}

[vagrant@localhost java]$ java MyApp
NG!
1

例外処理

public class MyApp {

	public static void div(int a, int b){
		System.out.println(a / b);
	}

	public static void main(String[] args){
		div(3, 0);
	}
}

[vagrant@localhost java]$ java MyApp
Exception in thread “main” java.lang.ArithmeticException: / by zero
at MyApp.div(MyApp.java:13)
at MyApp.main(MyApp.java:17)

例外処理

class MyException extends Exception {
	public MyException(String s){
		super(s);
	}
}

public class MyApp {

	public static void div(int a, int b){

		try {
			if (b < 0){
				throw new MyException("not minux!");
			}
			System.out.println(a / b);
		} catch (ArithmeticException e){
			System.err.println(e.getMessage());
		} catch(MyException e){
			System.err.println(e.getMessage());
		}		
	}
	public static void main(String&#91;&#93; args){
		div(3, 0);
		div(5, -2);
	}
}
&#91;/java&#93;

Wrapper Class
&#91;java&#93;
public class MyApp {

	public static void main(String&#91;&#93; args){
		// Integer i = new Integer(32);
		// int n = i.intValue();
		Integer i = 32; // auto boxing
		i = null;
		int n = i; // auto unboxing
		System.out.println();
	}
}
&#91;/java&#93;

&#91;vagrant@localhost java&#93;$ javac MyApp.java
&#91;vagrant@localhost java&#93;$ java MyApp
Exception in thread "main" java.lang.NullPointerException
        at MyApp.main(MyApp.java:14)

型を後程指定するジェネリック
&#91;java&#93;
class MyData<T> {
	public void getThree(T x){
		System.out.println(x);
		System.out.println(x);
		System.out.println(x);
	}
}

public class MyApp {

	public static void main(String[] args){
		MyData<Integer> i = new MyData<>();
		i.getThree(32);
	}
}

[vagrant@localhost java]$ javac MyApp.java
[vagrant@localhost java]$ java MyApp
32
32
32