Java的封装类和装箱拆箱详解

public class Test {
    public static void main(String[] args) {
        int num = 1;
        Object obj = new Num(num);//父类型引用指向子类型对象
        System.out.println(obj);
    }
}

public class Num {
    int num;
 
    public Num(int num) {
        this.num = num;
    }
 
    public String toString() {
        return "" + num;
    }
}
//实现封装

public class Test {
    public static void main(String[] args) {
        int num = 10;
 
        Integer num1 = num; // 自动装箱
        int num2 = num1; // 自动拆箱
    }
}

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

    public int intValue() {
        return value;
    }

public class Main {
    public static void main(String[] args) {
        Integer num1 = 100;
        Integer num2 = 100;
        Integer num3 = 200;
        Integer num4 = 200;
        System.out.println(num1 == num2);
        System.out.println(num3 == num4);
    }
}
/*
 * 运行结果
 * true
 * false
 */

private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];
 
        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;
 
            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
 
            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }
 
        private IntegerCache() {}
    }

public class Main {
    public static void main(String[] args) {
        Double num1 = 100.0;
        Double num2 = 100.0;
        Double num3 = 200.0;
        Double num4 = 200.0;
        System.out.println(num1 == num2);
        System.out.println(num3 == num4);
    }
}
/*
 * 运行结果
 * false
 * false
 */

    public static Double valueOf(double d) {
        return new Double(d);
    }