学习Java类必须知道的几点

class ParentClass {
	public static int  a=2;
	public int b=3;
	{
		System.out.println("this is anonymity b="+b);
	}
	static {
		a=4;
		System.out.println("this is static and a="+a);
	}
	public ParentClass() {
		System.out.println("this is parent gozao");
		this.s();
	}
	public void s() {
		System.out.println("this is parent");
	}
}
public class Son extends ParentClass {
	public Son(){
		System.out.println("this is  son gozao");
	}
	public static void main(String[] args) {
		ParentClass d = new Son();
		d.s();
	}
	public void s() {
		//super.s();
		System.out.println("this is son");
	}
}

this is static and a=4
this is anonymity 
this is parent gozao
this is son
this is  son gozao
this is son

2、不可变类  

1. 对于一般成员都是private，还可以使用public static final 来定义一个全局的常量。
2. 不提供对成员的修改方法，例如：setXXX()
3. 确保所有的方法不会被重载。手段有两种：使用final Class(强不可变类)，或者将所有的类方法加上final关键字(弱不可变类)。
4. 如果某一个类成员不是原始变量(primitive)或者不可变类，必须通过在成员初始化或者get方法时通过深度clone方法，来确保类的不可变。

public final class MyImmutableWrong {  // 使用final关键字声明为强不可变类
	private final int[] myArray;

	public MyImmutableWrong(int[] anArray) {
		this.myArray = anArray; // wrong
	}

	public String toString() {
		StringBuffer sb = new StringBuffer("Numbers are: ");
		for (int i = 0; i < myArray.length; i++) {
			sb.append(myArray[i] + " ");
		}
		return sb.toString();
	}
}

public class dd {
	public static void main(String[] args) {
         int array[]={1,2,3,4};
         MyImmutableWrong service=new MyImmutableWrong(array);
         array[2]=99;
         System.out.println(service.toString());
	}

}

public final class MyImmutableCorrect {
	private final int[] myArray;

	public MyImmutableCorrect(int[] anArray) {
		this.myArray = anArray.clone(); 
	}

	public String toString() {
		StringBuffer sb = new StringBuffer("Numbers are: ");
		for (int i = 0; i < myArray.length; i++) {
			sb.append(myArray[i] + " ");
		}
		return sb.toString();
	}
}

3、静态内部类  

public class Author {

  private final String name;

  public String getName() {
    return name;
  }

  public Author(String name_) {
    name = name_;
  }
  @Override
  public String toString() {
    return "Author [name=" + name + "]";
  }
}

public final class Update {  // 强不可变类

	private final Author author;  // 作者,是个引用变量
	private final String updateText;  // 更新内容
	private final long createTime;  // 更新时间
	
	// 私有构造函数，防止外部实例化
	private Update(Builder b_) {
		author = b_.author;
		updateText = b_.updateText;
		createTime = b_.createTime;
	}
	// 构建器
	public static class Builder {
		private long createTime;
		private Author author;
		private String updateText;

		public Builder author(Author author_) {
			author = author_;
			return this;
		}

		public Builder updateText(String updateText_) {
			updateText = updateText_;
			return this;
		}

		public Builder createTime(long createTime_) {
			createTime = createTime_;
			return this;
		}

		public Update build() {// 更新外部类的值
			return new Update(this);
		}
	}
	public Author getAuthor() {
		return author;
	}

	public String getUpdateText() {
		return updateText;
	}
	
	 @Override
	  public String toString() {
	    return "Update [author=" + author + ", updateText=" + updateText
	        + ", createTime=" + createTime + "]";
	  }
}

public class Test {

	public static void main(String[] args) {
		final Update first = getUpdate("abc");// 获取不可变类的实例
		System.out.println(first.toString());
	}

	private static Update getUpdate(String s) {
		Update.Builder b = new Update.Builder();
		b.updateText(s).author(new Author("mazhi"));
		return b.build();// 初始化不可变类并返回实例
	}
}

Update.Builder b = new Update.Builder(); // 获取静态内部类的实例对象