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Java装箱与拆箱详解(附实例代码)
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前言：
要理解装箱和拆箱的概念，就要理解Java数据类型
装箱：把基本类型用它们相应的引用类型包装起来，使其具有对象的性质。int包装成Integer、float包装成Float
拆箱：和装箱相反，将引用类型的对象简化成值类型的数据
Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i))
int b = new Integer(100); 这是自动拆箱
看下面一段代码
m1
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m11();
dt.m12();
}
public void m11() {
Integer a = new Integer(100);
Integer b = 100;
System.out.println("m11 result " + (a == b));
}
public void m12() {
Integer a = new Integer(128);
Integer b = 128;
System.out.println("m12 result " + (a == b));
}
}
打印结果是什么？
m11 result false
m12 result false
“==”比较的是地址，而a和b两个对象的地址不同，即是两个对象，所以都是false
通过javap解析字节码，内容如下
public void m11();
Code:
0: new #44; //class java/lang/Integer
3: dup
4: bipush 100
6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
9: astore_1
10: bipush 100
12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
15: astore_2
16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
19: new #59; //class java/lang/StringBuilder
22: dup
23: ldc #61; //String m11 result
25: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
28: aload_1
29: aload_2
30: if_acmpne 37
33: iconst_1
34: goto 38
37: iconst_0
38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
47: return
public void m12();
Code:
0: new #44; //class java/lang/Integer
3: dup
4: sipush 128
7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
10: astore_1
11: sipush 128
14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
17: astore_2
18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
21: new #59; //class java/lang/StringBuilder
24: dup
25: ldc #82; //String m12 result
27: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
30: aload_1
31: aload_2
32: if_acmpne 39
35: iconst_1
36: goto 40
39: iconst_0
40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
49: return
</init></init></init></init>
m2
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m21();
dt.m22();
}
public void m21() {
Integer a = new Integer(100);
Integer b = new Integer(100);
System.out.println("m21 result " + (a == b));
}
public void m22() {
Integer a = new Integer(128);
Integer b = new Integer(128);
System.out.println("m22 result " + (a == b));
}
}
打印结果是
m21 result false
m22 result false
a和b仍是两个对象
javap解析内容
public void m21();
Code:
0: new #44; //class java/lang/Integer
3: dup
4: bipush 100
6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
9: astore_1
10: new #44; //class java/lang/Integer
13: dup
14: bipush 100
16: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
19: astore_2
20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
23: new #59; //class java/lang/StringBuilder
26: dup
27: ldc #84; //String m21 result
29: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
32: aload_1
33: aload_2
34: if_acmpne 41
37: iconst_1
38: goto 42
41: iconst_0
42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
51: return
public void m22();
Code:
0: new #44; //class java/lang/Integer
3: dup
4: sipush 128
7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
10: astore_1
11: new #44; //class java/lang/Integer
14: dup
15: sipush 128
18: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
21: astore_2
22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
25: new #59; //class java/lang/StringBuilder
28: dup
29: ldc #86; //String m22 result
31: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
34: aload_1
35: aload_2
36: if_acmpne 43
39: iconst_1
40: goto 44
43: iconst_0
44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
53: return
m3
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m31();
dt.m32();
}
public void m31() {
Integer a = 100;
Integer b = 100;
System.out.println("m31 result " + (a == b));
}
public void m32() {
Integer a = 128;
Integer b = 128;
System.out.println("m32 result " + (a == b));
}
}
打印结果
m31 result true
m32 result false
为什么有第一个是true，第二个是false呢？观察javap解析的数据
javap解析内容
public void m31();
Code:
0: bipush 100
2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
5: astore_1
6: bipush 100
8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
11: astore_2
12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
15: new #59; //class java/lang/StringBuilder
18: dup
19: ldc #88; //String m31 result
21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
24: aload_1
25: aload_2
26: if_acmpne 33
29: iconst_1
30: goto 34
33: iconst_0
34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
43: return
public void m32();
Code:
0: sipush 128
3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
6: astore_1
7: sipush 128
10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
13: astore_2
14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
17: new #59; //class java/lang/StringBuilder
20: dup
21: ldc #90; //String m32 result
23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
26: aload_1
27: aload_2
28: if_acmpne 35
31: iconst_1
32: goto 36
35: iconst_0
36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
45: return
m4
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m41();
dt.m42();
}
public void m41() {
Integer a = Integer.valueOf(100);
Integer b = 100;
System.out.println("m41 result " + (a == b));
}
public void m42() {
Integer a = Integer.valueOf(128);
Integer b = 128;
System.out.println("m42 result " + (a == b));
}
}
打印结果
m41 result true
m42 result false
javap解析内容
public void m41();
Code:
0: bipush 100
2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
5: astore_1
6: bipush 100
8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
11: astore_2
12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
15: new #59; //class java/lang/StringBuilder
18: dup
19: ldc #92; //String m41 result
21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
24: aload_1
25: aload_2
26: if_acmpne 33
29: iconst_1
30: goto 34
33: iconst_0
34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
43: return
public void m42();
Code:
0: sipush 128
3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
6: astore_1
7: sipush 128
10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
13: astore_2
14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
17: new #59; //class java/lang/StringBuilder
20: dup
21: ldc #94; //String m42 result
23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
26: aload_1
27: aload_2
28: if_acmpne 35
31: iconst_1
32: goto 36
35: iconst_0
36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
45: return
}
分析
javap是Java自带的一个工具，可以反编译，也可以查看Java编译器生成的字节码(上面代码只使用了javap -c DataType)，是分析代码的一个好工具，具体怎么使用请Google一下
先看一下m4，为什么运行结果中出现了“true”呢，true说明a、b是同一个对象。
但a对象是调用Integer.valueOf()生成的，b是通过自动装箱生成的对象，为什么会是同一个对象呢？再看一下字节码吧，毕竟Java程序是依靠虚拟机运行字节码实现的。
m41这个方法只适用了一次valueOf()，但字节码中出现了两次，说明自动装箱时也调用了valueOf()。
下面是valueOf()具体实现
/**
* 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);
}
在【-128，127】之间的数字，valueOf返回的是缓存中的对象，所以两次调用返回的是同一个对象。
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