原始的布尔到字符串连接/转换 [英] primitive-boolean To String concatenation/conversion

问题描述

是如何工作的呢？我似乎无法找到答案。

how does this work? I can't seem to find an answer.

boolean bool=true;
System.out.println("the value of bool is : " + true);
//or
System.out.println("the value of bool is : " + bool);

• 有哪些东西会在幕后？


• 如何布尔被铸造为String作为一个布尔值不能隐
  键入铸造？


• 为自动装箱/拆箱的参与？


• 仿若的toString方法（）或将String.valueOf（）参与以某种方式？


• What are the things that are going on behind the scene?
• how does the boolean gets casted to the String as a boolean cannot be implicitly type casted?
• Is Autoboxing/Unboxing involved?
• Are methods like toString() or String.valueOf() are involved in some way?

推荐答案

的确切规则在Java语言规范中都有明确规定，的§5.1.11。字符串转换

The exact rules are spelled out in the Java Language Specification, §5.1.11. String Conversion

根据这些规则，STR+布尔等价于：

According to those rules, "str" + bool is equivalent to:

"str" + new Boolean(bool).toString()

这表示，允许编译器在整体前pression是如何准确地评估相当大的自主权。从JLS §15.18.1。字符串连接运算符+ ：

That said, the compiler is permitted considerable leeway in how exactly the overall expression is evaluated. From JLS §15.18.1. String Concatenation Operator +:

这是实现可以选择一步执行转换和级联，以避免创建，然后丢弃中间String对象。为了提高重复字符串连接的性能，Java编译器可以使用的StringBuffer 类或类似的技术，减少了中间字符串的数量由一个前pression评价创建的对象。

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

有关基本类型，一个实现也可以通过直接从原始类型转换成字符串优化掉的包装对象的创建。

For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.

例如，用我的编译如下：

For example, with my compiler the following:

boolean bool = true;
System.out.println("the value of bool is : " + bool);

是完全等价于：

boolean bool = true;
System.out.println(new StringBuilder("the value of bool is : ").append(bool).toString());

他们导致相同的字节codeS：

They result in identical bytecodes:

Code:
   0: iconst_1      
   1: istore_1      
   2: getstatic     #59                 // Field java/lang/System.out:Ljava/io/PrintStream;
   5: new           #166                // class java/lang/StringBuilder
   8: dup           
   9: ldc           #168                // String the value of bool is : 
  11: invokespecial #170                // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
  14: iload_1       
  15: invokevirtual #172                // Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder;
  18: invokevirtual #176                // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  21: invokevirtual #69                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  24: return        

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