public class IntCacheDemo {
/**
* 一道经典的Java笔试面试题
*/
public static void main(String[] args) {
Integer a = 2013;
Integer b = 2013;
if (a == b) {
System.out.println("a==b");
} else if (a.equals(b)) {
System.out.println("a equals b");
}
Integer c = 24;
Integer d = 24;
if (c == d) {
System.out.println("c==d");
} else if (a.equals(b)) {
System.out.println("c equals d");
}
}
}
程序运行结果
a equals b
c==d
解释说明
Integer等包装类型是引用类型,引用类型的对象之间的 == 比较,是比较两个引用是否指向同一个对象.因此a == b不成立,a equals b 成立.
但是为什么C == D也成立?
因为JDK内部默认会对-128到127之间的整数,进行缓存,两个对象骑士是同一个,因此C和D这两个引用指向同一个对象.
/**
*缓存，为了支持从-128到127（包含）的自动包装类型的“对象同一性语义”，这是Java语言规范所规定的。
*缓存，在第一次使用的时候初始化。缓存的大小可以通过“-XX:AutoBoxCacheMax=<size>”选项控制。
*在VM初始化期间，java.lang.Integer.IntegerCache.high属性可以被设置和保存在私有的系统属性sun.misc.VM class中。
*/
/**(担心翻译不够准确，误导大众，特给出英文注释)
* 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 -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.
*/
// IntegerCache，一个内部类，注意它的属性都是定义为static final
private static class IntegerCache {
//缓存的下界，-128，不可变
static final int low = -128;
//缓存上界，暂为null
static final int high;
//缓存的整型数组
static final Integer cache[];
static {
// 缓存上届，可以通过JVM属性来配置
int h = 127;
String integerCacheHighPropValue = sun.misc.VM
.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
//最大的数组值是Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low));
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for (int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
}
private IntegerCache() {
}
}
//128到127之间的是有缓存的
public static Integer valueOf(int i) {
assert IntegerCache.high >= 127;
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
为什么上界high是可以配置的,而下届low却不能?
类似的内部缓存还有
private static class ByteCache {
private ByteCache(){}
static final Byte cache[] = new Byte[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Byte((byte)(i - 128));
}
}
private static class ShortCache {
private ShortCache(){}
static final Short cache[] = new Short[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Short((short)(i - 128));
}
}
private static class LongCache {
private LongCache(){}
static final Long cache[] = new Long[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Long(i - 128);
}
}
private static class CharacterCache {
private CharacterCache(){}
static final Character cache[] = new Character[127 + 1];
static {
for (int i = 0; i < cache.length; i++)
cache[i] = new Character((char)i);
}
}
代码示例
public class AllCacheDemo {
/**
* 演示JDK内部缓存
*/
public static void main(String[] args) {
Integer a = 28;
Integer b = 28;
println(a == b);
Byte c = 25;
Byte d = 25;
println(c==d);
Short p=12;
Short q=12;
println(p==q);
Long x=127L;
Long y=127L;
println(x==y);
Character m='M';
Character n='M';
println(m==n);
}
public static void println(Object o){
System.out.println(o);
}
}
