一：“==”的不同含义

当“==”施加于原始数据类型变量时，是比较变量所保存的数据是否相等
当“==”施加于引用类型变量时，是比较这两个变量是否引用同一对象。

测试代码如下：

package test;

public class qq {

	public static void main(String[] args) {

		// TODO Auto-generated method stub

		Foo obj1=new Foo();

		Foo obj2=new Foo();

		System.out.println(obj1==obj2);

	}

}

class Foo{

	int value=100;

}

结果是false。

分析：例子中含义是判断这两个变量是否引用同一对象，因为例子中定义的是两个不同的对象，所以返回false。

二：类的构造方法

如果类提供了一个自定义的构造方法，将导致系统不再提供默认构造方法。
源代码：

package test;

public class qq {

	public static void main(String[] args) {

		// TODO Auto-generated method stub

		Foo obj1=new Foo();

	}

}

class Foo{

	int valud;

	public Foo(int initValue)

	value=initValue;

}

分析:程序报错，原因因为自定义了构造方法，系统不再提供默认构造方法。

三：静态初始化块的执行顺序

源代码：

class Root

{

	static{

		System.out.println("Root的静态初始化块");

	}

	{

		System.out.println("Root的普通初始化块");

	}

	public Root()

	{

		System.out.println("Root的无参数的构造器");

	}

}

class Mid extends Root

{

	static{

		System.out.println("Mid的静态初始化块");

	}

	{

		System.out.println("Mid的普通初始化块");

	}

	public Mid()

	{

		System.out.println("Mid的无参数的构造器");

	}

	public Mid(String msg)

	{

		//通过this调用同一类中重载的构造器

		this();

		System.out.println("Mid的带参数构造器，其参数值：" + msg);

	}

}

class Leaf extends Mid

{

	static{

		System.out.println("Leaf的静态初始化块");

	}

	{

		System.out.println("Leaf的普通初始化块");

	}

	public Leaf()

	{

		//通过super调用父类中有一个字符串参数的构造器

		super("Java初始化顺序演示");

		System.out.println("执行Leaf的构造器");

	}

}

public class TestStaticInitializeBlock

{

	public static void main(String[] args)

	{

		new Leaf();

	}

}

结果截图：

aaarticlea/png;base64,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" alt="" />

分析：

首先执行父类静态的内容，父类静态的内容执行完毕后，接着去执行子类的静态的内容，当子类的静态内容执行完毕之后，再去看父类有没有非静态代码块，如果有就执行父类的非静态代码块，父类的非静态代码块执行完毕，接着执行父类的构造方法；父类的构造方法执行完毕之后，它接着去看子类有没有非静态代码块，如果有就执行子类的非静态代码块。子类的非静态代码块执行完毕再去执行子类的构造方法。总之一句话，静态代码块内容先执行，接着执行父类非静态代码块和构造方法，然后执行子类非静态代码块和构造方法。

注意:子类的构造方法，不管这个构造方法带不带参数，默认的它都会先去寻找父类的不带参数的构造方法。如果父类没有不带参数的构造方法，那么子类必须用supper关键子来调用父类带参数的构造方法，否则编译不能通过。

四：在静态方法中访问类的实例成员

源代码：

package test;

public class qq

 {

  int x = 3;//类的实例变量，初始化值为3

  static int  y = 4;//类的静态变量，初始化值为4

  public static void method()//静态方法

  {

    System.out.println("实例变量x = " + new qq().x);//在静态方法中访问类的实例变量需首先进行类的实例化

    System.out.println("静态变量y = " + y);//在静态方法中可直接访问类的静态变量

   }

  public static void main(String[] args)

     {

          qq.method();

         qq ex = new qq();

        System.out.println("x = " + ex.x);

    }

}

五：Java字段初始化的规律

源代码：

package test;

public class qq {

	public static void main(String[] args) {

		// TODO Auto-generated method stub

InitialzeBlockClass obj=new InitialzeBlockClass();

System.out.println(obj.field);

obj=new InitialzeBlockClass(300);

System.out.println(obj.field);

	}

}

class InitialzeBlockClass{

	{

		field=200;

	}

	public int field=100;

	public InitialzeBlockClass(int value){

		this.field=value;

	}

	public InitialzeBlockClass(){

	}

}

实验结果截图：

aaarticlea/png;base64,iVBORw0KGgoAAAANSUhEUgAAAGgAAAA3CAIAAABvppjUAAABN0lEQVR4nO3YzY2DMBQA4VeXC6IeV0MzLmb3kADGP0s0hyQr5rvFXNCIJzuOHyHx6Rf4rwwHGQ4yHNSEKzlFRCxrvxYRKZc/F++kDrcukXIpOdXh1mXveDwZLt5LP6rnEo+Y+4NHr+HizVyE23+sS0TK+fxN1ovvfOlv8FK4fTLrYW4W3/nS3+CFUY3TXD5HtV+8matw9fd0bAnDxXtpdtWTat/0ONLyAAwZDjIcZDjIcJDhIMNBp3DVQe7qyOY5brhactqCeK00Ng/X/bnyWqnWhNuGtfsH77VSY7I5bLPotdLMbFfdanitNDH/4o7NwWulgcm1Uj17HkdGPABDhoMMBxkOMhxkOMhwkOEgw0GGgwwHGQ4yHGQ4yHCQ4SDDQYaDDAcZDjIcZDjIcJDhIMNBhoMMB/0CtBRQe2cauX4AAAAASUVORK5CYII=" alt="" />

分析（Java字段初始化规律）：所有类的变量都默认初始化为null，比如String a; Integer b;数字类的原始数据类型默认初始化为0，比如int a; short b; char c;boolean默认初始化为false;

类加载时直接初始化静态字段;

类加载时调用静态方法初始化静态字段;

实例化对象时,在调用构造函数之前代码块中初始化字段；

实例化对象时,在调用构造函数之时初始化字段;