值传递&引用传递 经典题目

public static void main(String [] args){

Integer a=1,b=2;

System.out.println("a = " + a +",b = " + b);

swap1(a,b);

System.out.println("a = " + a +",b = " + b);

}

版本1

public static void swap1(Integer a,Integer b){

Integer temp=b;

b=a;

a=temp;

}

如果传值是基本值，传的是原始值的副本

函数中修改，只是修改了副本，原始值不变

如果传的引用值，则是原始值的地址

怎么做？

Integer源码

/**

* The value of the {@code Integer}.

*

* @serial

*/

private final int value;

修改这个成员变量的值，修改地址

版本2

public static void swap2(Integer a,Integer b) throws Exception{

Field field = Integer.class.getDeclaredField("value"); //通过反射拿到成员变量

field.setAccessible(true); //设置访问权限 访问私有变量

int temp = a.intValue();

field.set(a,b.intValue());

field.set(b,temp);

}

为什么还是没有达到预期的效果？？？  a和b的值还是没有交换  

field.set(a,b.intValue()); //这句话改变了a，也改变了temp

temp和a用的一块内存地址，a改了temp也改了

Integer -128 - 127会有缓存

/**

* Cache to support the object identity semantics of autoboxing for values between

* -128 and 127 (inclusive) as required by JLS.

*

* The cache is initialized on first usage. The size of the cache

* may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.

* During VM initialization, java.lang.Integer.IntegerCache.high property

* may be set and saved in the private system properties in the

* sun.misc.VM class.

*/

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() {}

}

验证缓存的存在

public static void main(String [] args)throws Exception{

Integer a=1,b=1;

System.out.println(a==b);

}

返回true则说明是a,b在同一块地址

版本3

public static void swap3(Integer a,Integer b) throws Exception{

Field field = Integer.class.getDeclaredField("value"); //通过反射拿到成员变量

field.setAccessible(true); //设置访问权限 访问私有变量

Integer temp = new Integer(a.intValue()); //修改 new Integer对象 开辟新空间

field.set(a,b.intValue());

field.set(b,temp);

}

考点：

1,值传递与引用传递

2,自动装箱与拆箱

3,Integer -128 - 127 有cache

4,通过反射修改private final成员变量(注意修改访问权限)