Why can't we define a variable immediately after statement statement

Question

public class Foo{    
  public static void main(String []args){  
    int a=10;
    if(a==10)
    int x=20;    
  }
}

When compiling the above code, an error occurs during compilation.

But in the code below, this is normal. Why?

public class Foo{    
  public static void main(String []args){    
    int a=10;    
    if(a==10){    
    int x=20;
    }
  }
}

I do not understand. What exactly is going on in them? As far as I understand, we can write one statement after the if statement without curly braces (a compound statement block).

Answer 1

For clarity, the problem is here:

if(a==10)
    int x=20;

... and a mistake error: variable declaration not allowed here.

This is not allowed because there is a specific list of operators that are allowed after if; they are described in detail in JLS§14.5 :

• StatementWithoutTrailingSubstatement
• LabeledStatement
• IfThenStatement
• IfThenElseStatement
• WhileStatement
• ForStatement

StatementWithoutTrailingSubstatement (the first thing mentioned above) includes:

• Block
• EmptyStatement
• ExpressionStatement
• AssertStatement
• SwitchStatement
• DoStatement
• BreakStatement
• ContinueStatement
• ReturnStatement
• SynchronizedStatement
• ThrowStatement
• TryStatement

, LocalVariableDeclarationStatement . LocalVariableDeclarationStatement Block, , .

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Answer 2

, , .

. . int x=20 - , if {} . , , undefined , "if" . "x" "if"

public class Foo {

    public static void main(String []args){

        int a=10;

        if(a==10)
            int x=20;

    }
}

, .

, .