Java中初始化数据域的四种方法

1.在构造器中设置值

/**
 * 
 */
package jry;

/**
 * @author freewill
 * 
 */
public class InitFiledByConstructor {
	int id;
	String name;

	public InitFiledByConstructor(int id, String name) {
		this.id = id;
		this.name = name;
	}

	public static void main(String[] args) {
		InitFiledByConstructor a = new InitFiledByConstructor(23, "Jordan");
		System.out.println(a.id + a.name);
	}
}

2.在声明中赋值，这个太简单了，代码就不上了
3.显示域初始化

/**
 * 
 */
package jry;

/**
 * @author freewill
 * @desc 显示域初始化
 * @see Java Core page122
 */
public class InitDemo {
	static int nextId = 0;
	int id = assignId();

	static int assignId() {
		int r = nextId;
		nextId++;
		return r;
	}

	/**
	 * @param args
	 * @desc Testcase
	 */
	public static void main(String[] args) {
		System.out.println(new InitDemo().id);
		System.out.println(new InitDemo().id);
	}
}

4.初始化块

/**
 * 
 */
package jry;

/**
 * @author freewill
 * @desc 初始化块 Core Java page124
 */
public class InitFiledByBlock {
	private static int nextId;
	private int id;
	private String name;
	private double salary;

	public InitFiledByBlock(String n, double s) {
		this.name = n;
		this.salary = s;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public double getSalary() {
		return salary;
	}

	public void setSalary(double salary) {
		this.salary = salary;
	}

	{
		id = nextId;
		nextId++;
	}

	/**
	 * @param args
	 * @desc Testcase
	 */
	public static void main(String[] args) {
		InitFiledByBlock c = new InitFiledByBlock("Jordan", 2300);
		System.out.println(c.getId() + c.getName() + c.getSalary());
		InitFiledByBlock d = new InitFiledByBlock("James", 2000);
		System.out.println(d.getId() + d.getName() + d.getSalary());
	}
}

补充知识：调用构造器的具体处理步骤
·所有数据域被初始化为默认值(0、false、null)
·按照在类声明中出现的次序，依次执行所有域初始化语句和初始化块
·如果构造器第一行调用了第二个构造器，则执行第二个构造器主体
·执行这个构造器的主题
see Core Java page125