+ operator and strings | 易学教程

问题

I'm just starting out in AP Comp sci in high school and I stumbled across a question regarding the + operator in strings

Why does System.out.println ("number" + 6 + 4 * 5) result in number620

whereas

String s = "crunch";
int a = 3, b = 1;
System.out.print(s + a + b);
System.out.print(b + a + s);

result in crunch314crunch?

Thanks

回答1:

Depends on It's precedence order

When two operators share an operand the operator with the higher precedence goes first. For example, 1 + 2 * 3 is treated as 1 + (2 * 3), whereas 1 * 2 + 3 is treated as (1 * 2) + 3 since multiplication has a higher precedence than addition (+).

回答2:

If you want to do any Math into System.out.println, wrap it with braces, because Java sees String at 1st place.

Try System.out.println ("number" + (6 + 4 * 5)).

For 2nd example use: System.out.print(s + (a + b));

in this case you have sum of a and b.

but in System.out.print(b + a + s); b and a stay at the 1st place. Compiler does a+b 1st and after add String, you don't need braces

回答3:

"*" has a higher operator precedence than "+", this means the expression "4 * 5" is calculated before the String concatenation happens.

See http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

回答4:

It has something to do with the operator precedence:

First, 4 * 5 = 20

Second, "number" is concatenated with 6, which is further concatenated with 20.

回答5:

On your first example, since you used a multiplicative operator, 4 is being multiplied to 5 before concatenated to other string.
For the second example, you started with 2 integer before the String which will be calculated first before concatenated to a String.

回答6:

Why does System.out.println ("number" + 6 + 4 * 5) result in number620

Because, * has higher precedence than +, so the result is number620.

String s = "crunch"; int a = 3, b = 1; System.out.print(s + a + b); System.out.println(b + a + s); result in crunch314crunch?

Here, + is used as operator overloading not as binary operation. So, '+' do concat operation, not sum operation. So, the result is crunch314crunch.

回答7:

It's about two things:

operator precedence
string concatenation vs addition

The + has the following rules:

int + int => int
int + string => String 
String + int => String 
String + String => String

That is, as soon as a String is involved, + means concatenation.

Operators with the same precedence are evaluated left to right. Therefore

String + int + int => String + int => String

but

int + int + String => int + String => String

The first case uses concatenation only, whereas the second uses addition in the first step.

In your first example, * has higher precedence than +, so the multiplication is performed first.

String + int * int => String + int => String

回答8:

It's all about operator precedence and their associativity.

Your first example: "number" + 6 + 4 * 5

Acc. to operator precedence * is calculated first, so it becomes "number" + 6 + 20

Now, Associativity for + is Left -> Right (L->R), so + becomes a concatenation operator cause it is used with String, so the expression becomes "number6" + 20, and then "number620"

(Actually the int are converted to String before concatenation)

Similarly, your 2nd example:

Only + operator and start execution from L->R

"crunch" + 3 + 1 = "crunch3" + 1 = "crunch31"

1 + 3 + "crunch" = 4 + "crunch" = "4crunch"

回答9:

According to your question and answer

explanation is

1.
  A)System.out.println ("number" + 6 + 4 * 5);
  B)System.out.println ("number6" + 4 * 5);
  C)System.out.println ("number6" + 20);
  D)System.out.println ("number620");

And it prints output like

number620

And Second one is

2.
  A)System.out.print("crunch" + 3 + 1);
    System.out.print(1 + 3 + "crunch");
  B)System.out.print("crunch3" + 1);
    System.out.print(4 + "crunch");
  C)System.out.print("crunch31");
    System.out.print("4crunch");

And it prints output with in a line, why because you have used print() statement

crunch314crunch

来源：https://stackoverflow.com/questions/18757312/operator-and-strings

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java

string

operator-keyword