赋值的左边必须是变量

Question

Why doesn't this work?

private List<Integer> xShot = new ArrayList<Integer>();
     ...codes
     ...codes
     ...codes
     ...codes
     xShot.get(0) += 5;

Can't understand why the left-hand side of an assignment´isn't is a variable..

Someone help?

Answer 1

xShot.get(0) is a method call that returns a value. A variable is something you declare with a type that holds a value, like int x; , String name; , or List<Integer> xShot from your example. Those are the only things in Java that you can assign a value to using an assignment operator .

Answer 2

Although xShot.get(0) is a number, it is not a variable. You need to provide a variable for this to work. That said

int i = xShot.get(0);
i += 5;

Will not work. i will be incremented by 5, but xShot's object in location 5 is not the same object. You need to get, modify, and set the variable.

For example:

xShot.set(0, xShot.get(0) + 5);

Answer 3

xShot.get(0) returns an object; it isn't a variable , so you can't assign to it.

Also, Integer is immutable (you can't change its value), so you would have to replace the object at position 0 with a new Integer that has the calculated value.

You can achieve the intention of that line like this:

xShot.set(0, xShot.get(0) + 5);

Answer 4

It is like saying in Java:

5 = 6; // "Assign 5 to 6"

The left side ( 5 ) isn't variable.

Why is this example statement relevant? Because of Java uses always "pass by value". Which means that the return value of a method is also "return by value". This is pure mathematical: you can't change a value, you can change a variable. The same for Java. Five can never become six.
In other words: Only a value can be assigned to a variable.

So, the correct way of doing what you want is:

xShot.set(0, xShot.get(0) + 5);

Edit: In your situation: xShot.get(int) doesn't return a variable, but a value.

Answer 5

赋值的左侧必须明确地是一个变量，因为在您的statement情况下，表达式也可以是常量，如果允许的话将是错误的

Answer 6

If you just want to increment by 5 and aren't limited to List<Integer> specifically, you could avoid arguably verbose xShot.set(0, xShot.get(0) + 5) and do this instead:

List<AtomicInteger> xShot = new ArrayList<AtomicInteger>();
xShot.get(0).addAndGet(5);

This will increment the value of the AtomicInteger in xShot.get(0) by 5 in-place without further ado.