今天在做一个题目时,发现一个奇怪的Integer方法。
public class TestingString{
public static void main(String[] args){
TestingString inst_test = new TestingString();
int i1 = 128;
int i2 = 128;
int i3 = 2;
int i4 = 2;
Integer Ithree = new Integer(2); // 1
Integer Ifour = new Integer(2); // 2
System.out.println( Ithree == Ifour );
inst_test.method( i3 , i4 );
inst_test.method( i1 , i2 );
}
public void method( Integer i , Integer eye ) {
System.out.println(i == eye );
}
}
result: false
true
false
i3 == i4 和 i1 == i2的结果竟然不同,接着又继续测试了一下,只要i的值大于127,输出结果就是false。又翻了一下K&B's book, 其中有一段话:
写道
为了节省内存,对于下列包装对象的两个实例,当它们的基本值相同时,他们总是==:
Boolean
Byte
Character, \u0000 - \u007f(7f是十进制的127)
Integer, -128 - 127
这儿的Integer有一个范围,以前还没注意这儿,于是看了下Integer类中的源代码,发现其中有这么一段:
private static class IntegerCache {
private IntegerCache(){}
static final Integer cache[] = new Integer[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Integer(i - 128);
}
}
/**
* Returns a <tt>Integer</tt> instance representing the specified
* <tt>int</tt> value.
* If a new <tt>Integer</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Integer(int)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param i an <code>int</code> value.
* @return a <tt>Integer</tt> instance representing <tt>i</tt>.
* @since 1.5
*/
public static Integer valueOf(int i) {
final int offset = 128;
if (i >= -128 && i <= 127) { // must cache
return IntegerCache.cache[i + offset];
}
return new Integer(i);
}
这儿的IntegerCache有一个静态的Integer数组,在类加载时就将-128 到 127 的Integer对象创建了,并保存在cache数组中,一旦程序调用valueOf 方法,如果i的值是在-128 到 127 之间就直接在cache缓存数组中去取Integer对象。
再看上面的程序,由于我们声明的i1, i2, i3, i4都是基本类型,在java5中,自动装箱会调用Integer.valueOf(i)来将这些基本类型包装成Integer,由于i3, i4 都是在 -128 到 127 之间,所以它们包装后的Integer对象都是从cache缓存中取出的,返回的是一个对象,所以当我们调用 i == eye 时,它们指向的是同一个cache缓存中取出的Integer对象。而 i1 i2的值大于127,valueOf(int i)方法将会为i1 i2 各自返回一个新Integer对象,所以打印出false。
再看其它的包装器:
Byte:(全部缓存)
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));
}
}
/**
* Returns a <tt>Byte</tt> instance representing the specified
* <tt>byte</tt> value.
* If a new <tt>Byte</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Byte(byte)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param b a byte value.
* @return a <tt>Byte</tt> instance representing <tt>b</tt>.
* @since 1.5
*/
public static Byte valueOf(byte b) {
final int offset = 128;
return ByteCache.cache[(int)b + offset];
}
Short(-128 - 127缓存):
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));
}
}
/**
* Returns a <tt>Short</tt> instance representing the specified
* <tt>short</tt> value.
* If a new <tt>Short</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Short(short)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param s a short value.
* @return a <tt>Short</tt> instance representing <tt>s</tt>.
* @since 1.5
*/
public static Short valueOf(short s) {
final int offset = 128;
int sAsInt = s;
if (sAsInt >= -128 && sAsInt <= 127) { // must cache
return ShortCache.cache[sAsInt + offset];
}
return new Short(s);
}
Long(-128 - 127缓存):
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);
}
}
/**
* Returns a <tt>Long</tt> instance representing the specified
* <tt>long</tt> value.
* If a new <tt>Long</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Long(long)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param l a long value.
* @return a <tt>Long</tt> instance representing <tt>l</tt>.
* @since 1.5
*/
public static Long valueOf(long l) {
final int offset = 128;
if (l >= -128 && l <= 127) { // will cache
return LongCache.cache[(int)l + offset];
}
return new Long(l);
}
Float(没有缓存):
/**
* Returns a <tt>Float</tt> instance representing the specified
* <tt>float</tt> value.
* If a new <tt>Float</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Float(float)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param f a float value.
* @return a <tt>Float</tt> instance representing <tt>f</tt>.
* @since 1.5
*/
public static Float valueOf(float f) {
return new Float(f);
}
Double(没有缓存):
/**
* Returns a <tt>Double</tt> instance representing the specified
* <tt>double</tt> value.
* If a new <tt>Double</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Double(double)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param d a double value.
* @return a <tt>Double</tt> instance representing <tt>d</tt>.
* @since 1.5
*/
public static Double valueOf(double d) {
return new Double(d);
}
Character(<= 127缓存):
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);
}
}
/**
* Returns a <tt>Character</tt> instance representing the specified
* <tt>char</tt> value.
* If a new <tt>Character</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Character(char)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param c a char value.
* @return a <tt>Character</tt> instance representing <tt>c</tt>.
* @since 1.5
*/
public static Character valueOf(char c) {
if(c <= 127) { // must cache
return CharacterCache.cache[(int)c];
}
return new Character(c);
}
同样对于垃圾回收器来说:
Integer i = 100;
i = null;//will not make any object available for GC at all.
这里的代码不会有对象符合垃圾回收器的条件,这儿的i虽然被赋予null,但它之前指向的是cache中的Integer对象,而cache没有被赋null,所以Integer(100)这个对象还是存在。
而如果i大于127或小于-128则它所指向的对象将符合垃圾回收的条件:
Integer i = 10000;
i = null;//will make the newly created Integer object available for GC.
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