Java program goes into if statement where it shouldnt [duplicate]

This question already has an answer here:

• How do I compare strings in Java? 23 answers

So I am new to java and tried this

start=input.next();

    if (start != "ja" && start != "nein") {
        while (start != "ja" && start != "nein") {
            System.out.println("Nur ja oder nein eingeben.");
            start=input.next();
        }
    }
        if(start=="ja"){
            playing=true;
        }
        else{
            playing=false;
        }

But in the next line where I wrote

while(playing){...}

It compiles fine, but when I enter "ja" or "nein" it goes into the if statement rather than skipping it, what did I do wrong there?