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Java 装箱与拆箱详解及实例代码

2017-3-7 12:51| 发布者: zhangjf| 查看: 1776| 评论: 0

摘要: 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 ...

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

  2. Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i))
  3. int b = new Integer(100); 这是自动拆箱
复制代码

看下面一段代码

m1

  2. public class DataType {
  4. public static void main(String args[]) {
  5. DataType dt = new DataType();
  6. dt.m11();
  7. dt.m12();
  9. }
  11. public void m11() {
  12. Integer a = new Integer(100);
  13. Integer b = 100;
  14. System.out.println("m11 result " + (a == b));
  15. }
  17. public void m12() {
  18. Integer a = new Integer(128);
  19. Integer b = 128;
  20. System.out.println("m12 result " + (a == b));
  21. }
  24. }
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　　打印结果是什么？

  2. m11 result false
  3. m12 result false
复制代码

“==”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

  2. public void m11();
  3. Code:
  4. 0: new #44; //class java/lang/Integer
  5. 3: dup
  6. 4: bipush 100
  7. 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  8. 9: astore_1
  9. 10: bipush 100
  10. 12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  11. teger;
  12. 15: astore_2
  13. 16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  14. 19: new #59; //class java/lang/StringBuilder
  15. 22: dup
  16. 23: ldc #61; //String m11 result
  17. 25: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  18. ng/String;)V
  19. 28: aload_1
  20. 29: aload_2
  21. 30: if_acmpne 37
  22. 33: iconst_1
  23. 34: goto 38
  24. 37: iconst_0
  25. 38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  26. ng/StringBuilder;
  27. 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  28. ang/String;
  29. 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  30. ring;)V
  31. 47: return
  33. public void m12();
  34. Code:
  35. 0: new #44; //class java/lang/Integer
  36. 3: dup
  37. 4: sipush 128
  38. 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  39. 10: astore_1
  40. 11: sipush 128
  41. 14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  42. teger;
  43. 17: astore_2
  44. 18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  45. 21: new #59; //class java/lang/StringBuilder
  46. 24: dup
  47. 25: ldc #82; //String m12 result
  48. 27: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  49. ng/String;)V
  50. 30: aload_1
  51. 31: aload_2
  52. 32: if_acmpne 39
  53. 35: iconst_1
  54. 36: goto 40
  55. 39: iconst_0
  56. 40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  57. ng/StringBuilder;
  58. 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  59. ang/String;
  60. 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  61. ring;)V
  62. 49: return
  63. </init></init></init></init>
  64. 　
复制代码

m2

  2. public class DataType {
  4. public static void main(String args[]) {
  5. DataType dt = new DataType();
  6. dt.m21();
  7. dt.m22();
  8. }
  10. public void m21() {
  11. Integer a = new Integer(100);
  12. Integer b = new Integer(100);
  13. System.out.println("m21 result " + (a == b));
  14. }
  16. public void m22() {
  17. Integer a = new Integer(128);
  18. Integer b = new Integer(128);
  19. System.out.println("m22 result " + (a == b));
  20. }
  23. }
复制代码

　　打印结果是

  2. m21 result false
  3. m22 result false
复制代码

a和b仍是两个对象

javap解析内容

  2. public void m21();
  3. Code:
  4. 0: new #44; //class java/lang/Integer
  5. 3: dup
  6. 4: bipush 100
  7. 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  8. 9: astore_1
  9. 10: new #44; //class java/lang/Integer
  10. 13: dup
  11. 14: bipush 100
  12. 16: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  13. 19: astore_2
  14. 20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  15. 23: new #59; //class java/lang/StringBuilder
  16. 26: dup
  17. 27: ldc #84; //String m21 result
  18. 29: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  19. ng/String;)V
  20. 32: aload_1
  21. 33: aload_2
  22. 34: if_acmpne 41
  23. 37: iconst_1
  24. 38: goto 42
  25. 41: iconst_0
  26. 42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  27. ng/StringBuilder;
  28. 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  29. ang/String;
  30. 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  31. ring;)V
  32. 51: return
  34. public void m22();
  35. Code:
  36. 0: new #44; //class java/lang/Integer
  37. 3: dup
  38. 4: sipush 128
  39. 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  40. 10: astore_1
  41. 11: new #44; //class java/lang/Integer
  42. 14: dup
  43. 15: sipush 128
  44. 18: invokespecial #46; //Method java/lang/Integer."<init>":(I)V
  45. 21: astore_2
  46. 22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  47. 25: new #59; //class java/lang/StringBuilder
  48. 28: dup
  49. 29: ldc #86; //String m22 result
  50. 31: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  51. ng/String;)V
  52. 34: aload_1
  53. 35: aload_2
  54. 36: if_acmpne 43
  55. 39: iconst_1
  56. 40: goto 44
  57. 43: iconst_0
  58. 44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  59. ng/StringBuilder;
  60. 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  61. ang/String;
  62. 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  63. ring;)V
  64. 53: return
复制代码

m3

  2. public class DataType {
  4. public static void main(String args[]) {
  5. DataType dt = new DataType();
  6. dt.m31();
  7. dt.m32();
  8. }
  10. public void m31() {
  11. Integer a = 100;
  12. Integer b = 100;
  13. System.out.println("m31 result " + (a == b));
  14. }
  16. public void m32() {
  17. Integer a = 128;
  18. Integer b = 128;
  19. System.out.println("m32 result " + (a == b));
  20. }
  23. }
复制代码

　　打印结果

  2. m31 result true
  3. m32 result false
复制代码

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

  2. public void m31();
  3. Code:
  4. 0: bipush 100
  5. 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  6. teger;
  7. 5: astore_1
  8. 6: bipush 100
  9. 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  10. teger;
  11. 11: astore_2
  12. 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  13. 15: new #59; //class java/lang/StringBuilder
  14. 18: dup
  15. 19: ldc #88; //String m31 result
  16. 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  17. ng/String;)V
  18. 24: aload_1
  19. 25: aload_2
  20. 26: if_acmpne 33
  21. 29: iconst_1
  22. 30: goto 34
  23. 33: iconst_0
  24. 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  25. ng/StringBuilder;
  26. 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  27. ang/String;
  28. 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  29. ring;)V
  30. 43: return
  32. public void m32();
  33. Code:
  34. 0: sipush 128
  35. 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  36. teger;
  37. 6: astore_1
  38. 7: sipush 128
  39. 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  40. teger;
  41. 13: astore_2
  42. 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  43. 17: new #59; //class java/lang/StringBuilder
  44. 20: dup
  45. 21: ldc #90; //String m32 result
  46. 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  47. ng/String;)V
  48. 26: aload_1
  49. 27: aload_2
  50. 28: if_acmpne 35
  51. 31: iconst_1
  52. 32: goto 36
  53. 35: iconst_0
  54. 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  55. ng/StringBuilder;
  56. 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  57. ang/String;
  58. 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  59. ring;)V
  60. 45: return
复制代码

m4

  2. public class DataType {
  4. public static void main(String args[]) {
  5. DataType dt = new DataType();
  6. dt.m41();
  7. dt.m42();
  8. }
  11. public void m41() {
  12. Integer a = Integer.valueOf(100);
  13. Integer b = 100;
  14. System.out.println("m41 result " + (a == b));
  15. }
  17. public void m42() {
  18. Integer a = Integer.valueOf(128);
  19. Integer b = 128;
  20. System.out.println("m42 result " + (a == b));
  21. }
  22. }
复制代码

　　打印结果

  2. m41 result true
  3. m42 result false
复制代码

javap解析内容

  2. public void m41();
  3. Code:
  4. 0: bipush 100
  5. 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  6. teger;
  7. 5: astore_1
  8. 6: bipush 100
  9. 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  10. teger;
  11. 11: astore_2
  12. 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  13. 15: new #59; //class java/lang/StringBuilder
  14. 18: dup
  15. 19: ldc #92; //String m41 result
  16. 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  17. ng/String;)V
  18. 24: aload_1
  19. 25: aload_2
  20. 26: if_acmpne 33
  21. 29: iconst_1
  22. 30: goto 34
  23. 33: iconst_0
  24. 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  25. ng/StringBuilder;
  26. 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  27. ang/String;
  28. 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  29. ring;)V
  30. 43: return
  32. public void m42();
  33. Code:
  34. 0: sipush 128
  35. 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  36. teger;
  37. 6: astore_1
  38. 7: sipush 128
  39. 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In
  40. teger;
  41. 13: astore_2
  42. 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream;
  43. 17: new #59; //class java/lang/StringBuilder
  44. 20: dup
  45. 21: ldc #94; //String m42 result
  46. 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la
  47. ng/String;)V
  48. 26: aload_1
  49. 27: aload_2
  50. 28: if_acmpne 35
  51. 31: iconst_1
  52. 32: goto 36
  53. 35: iconst_0
  54. 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la
  55. ng/StringBuilder;
  56. 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l
  57. ang/String;
  58. 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St
  59. ring;)V
  60. 45: return
  62. }
复制代码

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

  2. /**
  3. * Returns a <tt>Integer</tt> instance representing the specified
  4. * <tt>int</tt> value.
  5. * If a new <tt>Integer</tt> instance is not required, this method
  6. * should generally be used in preference to the constructor
  7. * {[url=home.php?mod=space&uid=17823]@LINK[/url] #Integer(int)}, as this method is likely to yield
  8. * significantly better space and time performance by caching
  9. * frequently requested values.
  10. *
  11. * @param i an <code>int</code> value.
  12. * @return a <tt>Integer</tt> instance representing <tt>i</tt>.
  13. * @since 1.5
  14. */
  15. public static Integer valueOf(int i) {
  16. final int offset = 128;
  17. if (i >= -128 && i <= 127) { // must cache
  18. return IntegerCache.cache[i + offset];
  19. }
  20. return new Integer(i);
  21. }
复制代码

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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