Java 装箱与拆箱详解及实例代码

2024-03-05 12:19:03 66

Java装箱与拆箱详解

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

Integera=100;这是自动装箱(编译器调用的是staticIntegervalueOf(inti))
intb=newInteger(100);这是自动拆箱

看下面一段代码

publicclassDataType{

publicstaticvoidmain(Stringargs[]){
DataTypedt=newDataType();
dt.m11();
dt.m12();

}

publicvoidm11(){
Integera=newInteger(100);
Integerb=100;
System.out.println("m11result"+(a==b));
}

publicvoidm12(){
Integera=newInteger(128);
Integerb=128;
System.out.println("m12result"+(a==b));
}


}

打印结果是什么？

m11resultfalse
m12resultfalse

“==”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

publicvoidm11();
Code:
0:new#44;//classjava/lang/Integer
3:dup
4:bipush100
6:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
9:astore_1
10:bipush100
12:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
15:astore_2
16:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
19:new#59;//classjava/lang/StringBuilder
22:dup
23:ldc#61;//Stringm11result
25:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
28:aload_1
29:aload_2
30:if_acmpne37
33:iconst_1
34:goto38
37:iconst_0
38:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
41:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
44:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
47:return

publicvoidm12();
Code:
0:new#44;//classjava/lang/Integer
3:dup
4:sipush128
7:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
10:astore_1
11:sipush128
14:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
17:astore_2
18:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
21:new#59;//classjava/lang/StringBuilder
24:dup
25:ldc#82;//Stringm12result
27:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
30:aload_1
31:aload_2
32:if_acmpne39
35:iconst_1
36:goto40
39:iconst_0
40:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
43:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
46:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
49:return
</init></init></init></init>

publicclassDataType{

publicstaticvoidmain(Stringargs[]){
DataTypedt=newDataType();
dt.m21();
dt.m22();
}

publicvoidm21(){
Integera=newInteger(100);
Integerb=newInteger(100);
System.out.println("m21result"+(a==b));
}

publicvoidm22(){
Integera=newInteger(128);
Integerb=newInteger(128);
System.out.println("m22result"+(a==b));
}


}

打印结果是

m21resultfalse
m22resultfalse

a和b仍是两个对象

javap解析内容

publicvoidm21();
Code:
0:new#44;//classjava/lang/Integer
3:dup
4:bipush100
6:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
9:astore_1
10:new#44;//classjava/lang/Integer
13:dup
14:bipush100
16:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
19:astore_2
20:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
23:new#59;//classjava/lang/StringBuilder
26:dup
27:ldc#84;//Stringm21result
29:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
32:aload_1
33:aload_2
34:if_acmpne41
37:iconst_1
38:goto42
41:iconst_0
42:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
45:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
48:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
51:return

publicvoidm22();
Code:
0:new#44;//classjava/lang/Integer
3:dup
4:sipush128
7:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
10:astore_1
11:new#44;//classjava/lang/Integer
14:dup
15:sipush128
18:invokespecial#46;//Methodjava/lang/Integer."<init>":(I)V
21:astore_2
22:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
25:new#59;//classjava/lang/StringBuilder
28:dup
29:ldc#86;//Stringm22result
31:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
34:aload_1
35:aload_2
36:if_acmpne43
39:iconst_1
40:goto44
43:iconst_0
44:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
47:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
50:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
53:return

publicclassDataType{

publicstaticvoidmain(Stringargs[]){
DataTypedt=newDataType();
dt.m31();
dt.m32();
}

publicvoidm31(){
Integera=100;
Integerb=100;
System.out.println("m31result"+(a==b));
}

publicvoidm32(){
Integera=128;
Integerb=128;
System.out.println("m32result"+(a==b));
}


}

打印结果

m31resulttrue
m32resultfalse

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

publicvoidm31();
Code:
0:bipush100
2:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
5:astore_1
6:bipush100
8:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
11:astore_2
12:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
15:new#59;//classjava/lang/StringBuilder
18:dup
19:ldc#88;//Stringm31result
21:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
24:aload_1
25:aload_2
26:if_acmpne33
29:iconst_1
30:goto34
33:iconst_0
34:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
37:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
40:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
43:return

publicvoidm32();
Code:
0:sipush128
3:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
6:astore_1
7:sipush128
10:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
13:astore_2
14:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
17:new#59;//classjava/lang/StringBuilder
20:dup
21:ldc#90;//Stringm32result
23:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
26:aload_1
27:aload_2
28:if_acmpne35
31:iconst_1
32:goto36
35:iconst_0
36:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
39:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
42:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
45:return

publicclassDataType{

publicstaticvoidmain(Stringargs[]){
DataTypedt=newDataType();
dt.m41();
dt.m42();
}


publicvoidm41(){
Integera=Integer.valueOf(100);
Integerb=100;
System.out.println("m41result"+(a==b));
}

publicvoidm42(){
Integera=Integer.valueOf(128);
Integerb=128;
System.out.println("m42result"+(a==b));
}
}

打印结果

m41resulttrue
m42resultfalse

javap解析内容

publicvoidm41();
Code:
0:bipush100
2:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
5:astore_1
6:bipush100
8:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
11:astore_2
12:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
15:new#59;//classjava/lang/StringBuilder
18:dup
19:ldc#92;//Stringm41result
21:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
24:aload_1
25:aload_2
26:if_acmpne33
29:iconst_1
30:goto34
33:iconst_0
34:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
37:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
40:invokevirtual#74;//Methodjava/io/PrintStream.println:(Ljava/lang/St
ring;)V
43:return

publicvoidm42();
Code:
0:sipush128
3:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
6:astore_1
7:sipush128
10:invokestatic#49;//Methodjava/lang/Integer.valueOf:(I)Ljava/lang/In
teger;
13:astore_2
14:getstatic#53;//Fieldjava/lang/System.out:Ljava/io/PrintStream;
17:new#59;//classjava/lang/StringBuilder
20:dup
21:ldc#94;//Stringm42result
23:invokespecial#63;//Methodjava/lang/StringBuilder."<init>":(Ljava/la
ng/String;)V
26:aload_1
27:aload_2
28:if_acmpne35
31:iconst_1
32:goto36
35:iconst_0
36:invokevirtual#66;//Methodjava/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
39:invokevirtual#70;//Methodjava/lang/StringBuilder.toString:()Ljava/l
ang/String;
42:invokevirtual#74;//Methodjava/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-cDataType)，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

/**
*Returnsa<tt>Integer</tt>instancerepresentingthespecified
*<tt>int</tt>value.
*Ifanew<tt>Integer</tt>instanceisnotrequired,thismethod
*shouldgenerallybeusedinpreferencetotheconstructor
*{@link#Integer(int)},asthismethodislikelytoyield
*significantlybetterspaceandtimeperformancebycaching
*frequentlyrequestedvalues.
*
*@paramian<code>int</code>value.
*@returna<tt>Integer</tt>instancerepresenting<tt>i</tt>.
*@since1.5
*/
publicstaticIntegervalueOf(inti){
finalintoffset=128;
if(i>=-128&&i<=127){//mustcache
returnIntegerCache.cache[i+offset];
}
returnnewInteger(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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Java 装箱与拆箱详解及实例代码

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