赋值表达式的编译错误

Question

How do I switch every two items in an array list?

For example: "hi", "how", "are", "you" becomes:

"how", "hi", "you", "are"

This is my error (in practice-it):

The compiler found a data type it was not expecting here. Sometimes this error occurs when you mistake = for == when comparing values

unexpected type
required: variable
found : value
list.get(i) = list.get(i+1);

unexpected type
required: variable
found : value
list.get(i+1) = temp;

This is what I have:

    public void switchPairs(ArrayList<String> list){
        String temp = "";
        for(int i = 0; i<= list.size(); i+2){
            temp = list.get(i);
            list.get(i) = list.get(i+1);
            list.get(i+1) = temp;
        }
    }

Answer 1

You are attempting to use the return of the get method as a variable. Unlike array access expressions, eg arr[i] = value , which are legal, the results of method calls cannot be used this way. You must use the set method instead.

temp = list.get(i);
list.set(i, list.get(i + 1));
list.set(i + 1, temp);

This will fix the compiler error, but running this will guarantee an IndexOutOfBoundsException .

If the list size is even, then list.get(i) will throw an IndexOutOfBoundsException when i reaches list.size() . Remember that valid indexes are from 0 through size() - 1 .

If the list size is odd, then list.get(i + 1) will throw an IndexOutOfBoundsException .

You must alter your for loop condition to stop before i and i + 1 go out of bounds. (The increment needs += to have an effect here.)

for(int i = 0; i < list.size() - 1; i+=2){

That will leave the last item in a list of odd size untouched.

Answer 2

Because every method return a value , not a variable .

and the left operand of an assignment operator must be a variable, or a compile-time error occurs.

You can think like this way: If the we can modify a variable via it's getter, that encapsulation makes nonsense, Since usually there is a getter for a private attribute.

In a word, in this case(and many other cases), you should use a setter to achieve your gold. Like this:

list.set(i, "something you want");