java - 在 Java 中创建的对象数量

Question

下面4种情况分别创建了多少个对象？

int[] array = new int[10];

String[]  str = new String[10];

Edit 1 : 

String[] str = { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };

Edit 2 : 


String[] str1 = new String[10]; 

    for(int i = 0;i<10;i++){
         str1[i] = "Str";
    }

Answer 1

Java 数组是单个对象。并且分配一个新数组只能分配那个对象。分配可以嵌套，方法是在数组初始化 block 中写入 new，自动装箱，或者通过对象构造函数也以链式操作的形式调用 new。

您的三个案例的答案是:1、1 和 1。下面有详细信息，以及一些突出特殊情况的其他示例。

字符串和自动装箱的整数也可以是特殊情况，因为 JVM 为它们提供了特殊的缓存。以下注释示例中的详细信息:

// 1 object, an array with 10 elements (set to zero)
int[] array = new int[10]; 

// 1 objects, an array with 10 elements (set to null)
String[]  str = new String[10];  

// 1 object, an array with 10 elements pointing at objects 
// that have been preallocated within the String pool.  See 
// the appendium below for evidence.
String[] str = { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" }; 

// 3 objects, one for the array and one per integer.
Integer[] a4 = new Integer[] { new Integer(1), new Integer(2) }; 

// 1 object again, Java has an Integer pool of limited size which is used
// to optimise auto boxing; 1 and 2 will definitely be within that default
// range
Integer[] a6 = new Integer[] { 1, 2 }; 

// 3 objects, the default size of the int pool is fairly low
// but it can be increased via a JVM flag.
Integer[] a5 = new Integer[] { 1000, 2000 }; 

// 3-5 objects -- 1 for the array, one for each of the string objects and 1 
// per char array backing the string.  Depending on JVM version the char 
// array may be shared with the interned strings, so that one is a little tricky
// and is why I said 3-5.
String[] str = {new String("1"), new String("2")};  

附录

只是为了好玩，下面是常量池在起作用的证据。

下面的 Java 代码编译成下面的字节码，注意只有数组被分配了。这些元素正在使用类池常量。我缩短了输出，它只是为每个元素重复相同的代码(dup、iconst、ldc、aastore，...)

java代码:

String[] str = { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" }; 

JVM 字节码:

   0: bipush        10
   2: anewarray     #2                  // class java/lang/String
   5: dup           
   6: iconst_0      
   7: ldc           #3                  // load constant from class pool - String 1
   9: aastore                           // store into array
  10: dup           
  11: iconst_1      
  12: ldc           #4                  // String 2
  14: aastore       
  15: dup           
  16: iconst_2