基本类型包装类及编译器优化（常量池）

1.Boolean

/**
 * 
 * @author Aruforce
 * Booelean 为不可变类,Java实现了 常量池机制。
 * 直接赋值'public static Boolean flag1 = true;' 会被优化成 
 * 'public static Boolean flag1 = Boolean.valueOf(true);'返回一个Boolean的固定地址
 */
public class TestBoolean {
    public static Boolean flag1 = true;//常量池
    public static Boolean flag2 = true;//常量池
    public static void main(String[] args) {
        System.out.println(flag1==flag2);//true
        Boolean flag3 = new Boolean(true);//堆内的对象
        System.out.println(flag1 == flag3);//false
        Boolean flag4 =true;//常量池
        System.out.println(flag1 == flag4);//true
        flag4 = new Boolean(false);//堆内的对象
        System.out.println(flag3 == flag4);//false
    }
}

2.Byte

/**
 * 
 * @author Aruforce
 * Byte为不可变类，Java为其实现了常量池机制
 * 直接赋值'public static Byte b1 = 1' -128-0-127 会被优化成 
 * 'Byte.valueOf(1)' 返回ByteCache.cache[]里面对应的值
 */
public class TestByte {
    public static Byte b1 = 1;//常量池
    public static Byte b2 = 1;//常量池
    public static void main(String[] args) {
        Byte b3 = 1;
        Byte b4 = new Byte((byte)1);
        System.out.println(b1 == b2);//true
        System.out.println(b1 ==b3);//true
        System.out.println(b1==b4);//false
    }
}

3.Charactor

/**
 * 
 * @author Aruforce
 * Character 为不可变类，Java为（Character c int值在127以下时）实现了常量池机制 
 * 直接赋值'public static Character character1 = 'A' 被编译器优化为
 * 'public static Character character1 = Character.valueOf('A')'
 * 
 */
public class TestCharactor {
    public static Character character1 = 'A';
    public static Character character2 = 'A';
    public static Character character11 = '中';
    public static void main(String[] args) {
        Character character3 = 'A';
        Character character4 = new Character('A');
        System.out.println(character1 == character2);//true
        System.out.println(character1 == character3);//true
        System.out.println(character1 == character4);//false
        Character character12 = '中';
        Character character13 = new Character('中');
        System.out.println(character11 == character12);//false
        System.out.println(character12 == character13);//false
    }
}

4.Double

/**
 * 
 * @author Aruforce
 * Double 是不可变类,但是Java未实现常量池机制，显然数值类型的也不需要
 * public static Double d1 = 1.0 优化为 public static Double d1 = Double.valueOf(1.0D);
 * 实际上也是调用 new Doubel();
 */
public class TestDouble {
    public static Double d1 = 1.0;
    public static Double d2 = 1.0;
    public static void main(String[] args) {
        Double d3 = 1.0;
        Double d4 = new Double(1.0);
        System.out.println(d1 == d2);//false
        System.out.println(d1 == d3);//false
        System.out.println(d1 == d4);//false
    }
}

5.Float

/**
 * 
 * @author Aruforce
 * Float 是不可变类,但是Java未实现常量池机制，显然数值类型的也不需要
 * public static Float f1 = 1.0f; 优化为 public static Float f1 = Float .valueOf(1.0f);
 * 实际上也是调用 new Float();
 */
public class TestFloat {
    public static Float f1 = 1.0f;
    public static Float f2 = 1.0f;
    public static void main(String[] args) {
        Float f3 = 1.0f;
        Float f4 = new Float(1.0f);
        System.out.println(f1 == f2);//false
        System.out.println(f1 == f3);//false
        System.out.println(f1 == f4);//false
    }
}

6.Integer

/**
 * 
 * @author Aruforce
 * {@link Integer} 是不可变类，Java为介于-128-0-127的实现了常量池
 * public static Integer i1 = 127;被编译器优化为
 * public static Integer i1 = Integer.valueOf(1);
 */
public class TestInteger {
    public static Integer i1 = 127;
    public static Integer i2 = 127;
    public static void main(String[] args) {
        Integer i3 = 127;
        Integer i4 = new Integer(127);
        System.out.println(i1 == i2);// true
        System.out.println(i1 == i3);// true
        System.out.println(i1 == i4);// false
    }
}

7.Long

/**
 * 
 * @author Aruforce
 * {@link Long} 是不可变类，Java为介于-128-0-127的实现了常量池
 * public static Long i1 = 127;被编译器优化为
 * public static Long i1 = Long.valueOf(1);
 */
public class TestLong {
    public static Long l1 = 127l;
    public static Long l2 = 127l;
    public static void main(String[] args) {
        Long l3 = 127l;
        Long l4 = new Long(127l);
        System.out.println(l1 == l2);// true
        System.out.println(l1 == l3);// true
        System.out.println(l1 == l4);// false
    }

}

8.Short

/**
 * 
 * @author Aruforce
 * {@link Short} 是不可变类，Java为介于-128-0-127的实现了常量池
 * public static Short i1 = 127;被编译器优化为
 * public static Short i1 = Short.valueOf(1);
 */
public class TestShort {
    public static Short s1 = 127;
    public static Short s2 = 127;
    public static void main(String[] args) {
        Short s3 = 127;
        Short s4 = new Short((short)127);
        System.out.println(s1 == s2);// true
        System.out.println(s1 == s3);// true
        System.out.println(s1 == s4);// false
    }
}