Explicit vs implicit call of toString

There’s little difference. Use the one that’s shorter and works more often.

If you actually want to get the string value of an object for other reasons, and want it to be null friendly, do this:

String s = String.valueOf(obj);

Edit: The question was extended, so I’ll extend my answer.

In both cases, they compile to something like the following:

System.out.println(new StringBuilder().append("obj: ").append(obj).toString());

When your toString() is implicit, you’ll see that in the second append.

If you look at the source code to java, you’ll see that StringBuilder.append(Object) looks like this:

public StringBuilder append(Object obj) {
    return append(String.valueOf(obj));
}

where String.valueOf looks like this:

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

Now, if you toString() yourself, you bypass a null check and a stack frame and go straight to this in StringBuilder:

public StringBuilder append(String str) {
    super.append(str);
    return this;
}

So…very similar things happens in both cases. One just does a little more work.