java - 为什么外循环不能加条件

Question

这是我的演示，很简单，但是我无法在外循环中添加添加i<10如果我尝试添加，它会提醒我:

This method must return a result of type String

public String testFor() {
    for(int i = 0; ; i++) {
        for(int j = 0; j < 10; j++) {
             return "success";
        }
    }
}

为什么会发生这种情况？

Answer 1

JLS的相关部分是Sec 8.4.7 Method body ;相关部分是:

If a method is declared to have a return type (§8.4.5), then a compile-time error occurs if the body of the method can complete normally (§14.1).

( "complete normally" 是一个有点令人困惑的术语；它基本上意味着在不执行 return 、 throw 或 System.exit 语句的情况下完成)

和Sec 14.21, Unreachable statements 。相关部分是:

A non-empty block that is not a switch block can complete normally iff the last statement in it can complete normally.

...

A basic for statement can complete normally iff at least one of the following is true:

• The for statement is reachable, there is a condition expression, and the condition expression is not a constant expression (§15.28) with value true.
• There is a reachable break statement that exits the for statement.

The contained statement is reachable iff the for statement is reachable and the condition expression is not a constant expression whose value is false.

内部循环实际上与行为无关:您将看到完全相同的行为:

public String testFor() {
  // Compiler error if you uncomment the condition.
  for(int i = 0; /* i<10 */; i++) { 
  }
}

如果没有条件，则两个条件都不成立，因此循环无法正常完成；因此，它无法到达循环之后。因此，没有 return (或 throw 或 System.exit )声明是必需的。

如果添加条件i<10 ，这不是编译时常量，因此循环可以正常完成。因此，您需要添加 return (或 throw 或 System.exit )循环后的语句。