Javac语法糖之增强for循环

Posted 2020-10-03

 

加强的for循环有两种，遍历数组和实现了Iterable接口的容器。javac通过visitForeachLoop()方法来实现解语法糖，代码如下：

 /** Translate away the foreach loop.  */
    public void visitForeachLoop(JCEnhancedForLoop tree) {
        if (types.elemtype(tree.expr.type) == null)
            visitIterableForeachLoop(tree);
        else
            visitArrayForeachLoop(tree);
    }

 

首先来看遍历数组。

A statement of the form

for ( T v : arrayexpr )
          stmt;

(where arrayexpr is of an array type) gets translated to

for ( { arraytype #arr = arrayexpr;
             int #len = array.length;
             int #i = 0; };
           #i < #len; i$++ ) {
         T v = arr$[#i];
         stmt;
     }

where #arr, #len, and #i are freshly named synthetic local variables.

举个例子，如下：

Integer[] array = { 1, 2, 3, 4, 5 };
for (Integer i : array) {
	System.out.println(i);
}

 

解语法糖后的结果如下：

for (Integer[] arr$ = array, len$ = arr$.length, i$ = 0; i$ < len$; ++i$) {
    Integer i = arr$[i$];
    {
        System.out.println(i);
    }
}

 

来看看对容器的遍历，如下：

 A statement of the form

for ( T v : coll ) 
          stmt ;

 (where coll implements Iterable<? extends T>) gets translated to

     for ( Iterator<? extends T> #i = coll.iterator(); #i.hasNext(); ) {
         T v = (T) #i.next();
         stmt;
     }

 where #i is a freshly named synthetic local variable.

List<String> list = new ArrayList<String>();
for(String str:list){
	System.out.println(str);
}　　

 解语法糖后的结果如下：

for (java.util.Iterator i$ = list.iterator(); i$.hasNext(); ) {
    String str = (String)i$.next();
    {
        System.out.println(str);
    }
}