Why is "while((String tmp=x))" not valid Java syntax?

Question

I have a clarification about some Java code:

What's the difference between these codes, that one can be compiled while the other cannot.

I'm not interested on "how to fix the error" because I've already solved it, but more on an explanation about the problem:

Working

public void  x(){
    HashMap<String , Integer> count= new HashMap<String, Integer>();
    Scanner scan= new Scanner("hdsh");

    String tmp;
    while((tmp=scan.next())!=null){
        if(count.containsKey(tmp)){
            count.put(tmp, 1);
        }
        else{
            count.put(tmp, count.get(tmp)+1);
        }
         tmp=scan.next();
    }

}

Not Working

public void  x(){
    HashMap<String , Integer> count= new HashMap<String, Integer>();
    Scanner scan= new Scanner("hdsh");

    while((String tmp=scan.next())!=null){
        if(count.containsKey(tmp)){
            count.put(tmp, 1);
        }
        else{
            count.put(tmp, count.get(tmp)+1);
        }
         tmp=scan.next();
    }

}

The errors of Eclipse are:

Multiple markers at this line:

• String cannot be resolved to a variable
• Syntax error on token "tmp", delete this token
• String cannot be resolved to a variable
• Syntax error on token "tmp", delete this token

Answer 1

You cannot declare a variable inside of an expression. (except for the first part of a for loop)

Answer 2

JLS §14.12:

WhileStatement:
    while ( Expression ) Statement

JLS §15.27

Expression:
    AssignmentExpression

JLS §15.26

AssignmentExpression:
    ConditionalExpression
    Assignment

Assignment:
    LeftHandSide AssignmentOperator AssignmentExpression

LeftHandSide:
    ExpressionName
    FieldAccess
    ArrayAccess

LeftHandSide cannot be a declaration, so it is not allowed.