什么是'？在代码中[重复]

Question

有人能解释一下这些代码吗？为什么里面有?？

package course.examples.theanswer;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class TheAnswer extends Activity {

public static final int[] answers = { 42, -10, 0, 100, 1000 };
public static final int answer = 42;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.answer_layout);

    TextView answerView = (TextView) findViewById(R.id.answer_view);


    int val = findAnswer();
    String output = (val == answer) ? "42" : "We may never know";
    answerView.setText("The answer to life, the universe and everything is:\n\n"
                    + output);
}

private int findAnswer() {
    for (int val : answers) {
        if (val == answer)
            return val;
    }
    return -1;
}
}

Answer 1

它的条件/三元运算符。

String output = (val == answer) ? "42" : "We may never know";

实际上等同于：

if(val==answer)
{
 output = "42" 
}

else
{
output = "we may never know"
}

Answer 2

这是一个三元运算符..

这一行：

String output = (val == answer) ? "42" : "We may never know";

和这个是一样的：

String output;
if (val == answer) {
  output = "42";
} else {
  output = "We may never know";
}

它通常用于简单的条件和赋值......单行。

Answer 3

?: 是 if-then-else 行为的运算符

(val == answer) ? "42" : "We may never know";

这意味着

if(val==answer) {
    output = "42"
} else { 
     output = "We may never know";
} 

Answer 4

如果你指的是这一行：

String output = (val == answer) ? "42" : "We may never know";
    answerView.setText("The answer to life, the universe and everything is:\n\n"
                    + output);

? 是 (if.​​..then...else) 的缩写形式。

if (val == answer) then output = "42" else output = "We may never know"; answerView.setText("生命、宇宙和一切的答案是：\n\n" + 输出）

Answer 5

在这条线上

String output = (val == answer) ? "42" : "We may never know";

那个？代表三元运算符：http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html

如果 val == answer 为真，则分配给输出的值将为“42”。如果它等于 false，它将被分配“我们可能永远不会知道”

val = answer;
...
String output = (val == answer) ? "42" : "We may never know";
System.out.println(output);

会输出：

42

而：

val = answer + 1;
...
String output = (val == answer) ? "42" : "We may never know";
System.out.println(output);

会输出：

我们可能永远不会知道

Answer 6

这称为Ternary operator。这是if-else 声明的一种替换。

(Condition) ? {if true execute this} : {else do this}