What is a Question Mark “?” and Colon “:” Operator

问题:

This question already has an answer here:

• What is the Java ?: operator called and what does it do? 15 answers

Two questions about using a question mark \"?\" and colon \":\" operator within the parentheses of a print function: What do they do? Also, does anyone know the standard term for them or where I can find more information on their use? I\'ve read that they are similar to an \'if\' \'else\' statement.

int row = 10;
int column;
while (row >= 1)
{
    column = 1;
    while(column <= 10)
    {
        System.out.print(row % 2 == 1 ? \"<\" : \"\\r>\");
        ++column;
    }
    --row;
    System.out.println();
}

回答1:

This is the ternary conditional operator, which can be used anywhere, not just the print statement. It\'s sometimes just called \"the ternary operator\", but it\'s not the only ternary operator, just the most common one.

Here\'s a good example from Wikipedia demonstrating how it works:

A traditional if-else construct in C, Java and JavaScript is written:

if (a > b) {
    result = x;
} else {
    result = y;
}

This can be rewritten as the following statement:

result = a > b ? x : y;

Basically it takes the form:

boolean statement ? true result : false result;

So if the boolean statement is true, you get the first part, and if it\'s false you get the second one.

Try these if that still doesn\'t make sense:

System.out.println(true ? \"true!\" : \"false.\");
System.out.println(false ? \"true!\" : \"false.\");

回答2:

Thats an if/else statement equilavent to

if(row % 2 == 1){
  System.out.print(\"<\");
}else{
  System.out.print(\"\\r>\");
}

回答3:

a=1;
b=2;

x=3;
y=4;

answer = a > b ? x : y;

answer=4 since the condition is false it takes y value.

A question mark (?)
. The value to use if the condition is true

A colon (:)
. The value to use if the condition is false

回答4:

Maybe It can be perfect example for Android, For example:

void setWaitScreen(boolean set) {
    findViewById(R.id.screen_main).setVisibility(
            set ? View.GONE : View.VISIBLE);
    findViewById(R.id.screen_wait).setVisibility(
            set ? View.VISIBLE : View.GONE);
}

回答5:

it is a ternary operator and in simple english it states \"if row%2 is equal to 1 then return < else return /r\"

回答6:

Also just though I\'d post the answer to another related question I had,

a = x ? : y;

Is equivalent to:

a = x ? x : y;

If x is false or null then the value of y is taken.

回答7:

They are called the ternary operator since they are the only one in Java.

The difference to the if...else construct is, that they return something, and this something can be anything:

  int k = a > b ? 7 : 8; 
  String s = (foobar.isEmpty ()) ? \"empty\" : foobar.toString ();