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Java 裝箱與拆箱詳解及實例代碼

 更新時間：2017年01月11日 09:49:32   投稿：lqh  
這篇文章主要介紹了Java 裝箱與拆箱詳解及實例代碼的相關資料,這里對java 的裝箱及拆箱進行了基本概念詳解及簡單使用，需要的朋友可以參考下

Java 裝箱與拆箱詳解

前言：

要理解裝箱和拆箱的概念，就要理解Java數(shù)據(jù)類型

裝箱：把基本類型用它們相應的引用類型包裝起來，使其具有對象的性質(zhì)。int包裝成Integer、float包裝成Float

拆箱：和裝箱相反，將引用類型的對象簡化成值類型的數(shù)據(jù)

Integer a = 100;         這是自動裝箱 (編譯器調(diào)用的是static Integer valueOf(int i))
int   b = new Integer(100); 這是自動拆箱

看下面一段代碼

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));
  }
 
   
}

　　打印結(jié)果是什么？

m11 result false
m12 result false

“==”比較的是地址，而a和b兩個對象的地址不同，即是兩個對象，所以都是false

通過javap解析字節(jié)碼，內(nèi)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));
  }
 
   
}

　　打印結(jié)果是

m21 result false
m22 result false

a和b仍是兩個對象

javap解析內(nèi)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));
  }
 
 
}

　　打印結(jié)果

m31 result true
m32 result false

為什么有第一個是true，第二個是false呢？觀察javap解析的數(shù)據(jù)

javap解析內(nèi)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));
  }
}

　　打印結(jié)果

m41 result true
m42 result false

javap解析內(nèi)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自帶的一個工具，可以反編譯，也可以查看Java編譯器生成的字節(jié)碼(上面代碼只使用了javap -c DataType)，是分析代碼的一個好工具，具體怎么使用請Google一下

先看一下m4，為什么運行結(jié)果中出現(xiàn)了“true”呢，true說明a、b是同一個對象?！?/p>

但a對象是調(diào)用Integer.valueOf()生成的，b是通過自動裝箱生成的對象，為什么會是同一個對象呢？再看一下字節(jié)碼吧，畢竟Java程序是依靠虛擬機運行字節(jié)碼實現(xiàn)的。

m41這個方法只適用了一次valueOf()，但字節(jié)碼中出現(xiàn)了兩次，說明自動裝箱時也調(diào)用了valueOf()。

下面是valueOf()具體實現(xiàn)

/**
 * 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】之間的數(shù)字，valueOf返回的是緩存中的對象，所以兩次調(diào)用返回的是同一個對象。

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