There are common questions which come up repeatedly in Java. Even if you know the answer, it's worth getting a more thorough understanding of what is happening in these cases.

How do I compare Strings?

The more general question is, how do I compare the contents of an Object? What is surprising when you use Java for the first time is that if you have a variable like String which is a reference to an Object, not the Object itself. This means when you use == you are only comparing references. Java has no syntactic sugar to hide this fact, so == only compares references, not the contents of references.

If you’re in any doubt, Java only has primitives and references for data types up to Java 9 (in Java 10 it might value value types). The other type is void, which is only used as a return type.

Some other questions you should check out:

• Is Java Pass by Reference or Pass by Value?
• How do I compare Strings?
• Why does 128 == 128 return false but 127 == 127 return true?

How do I avoid lots of != null?

Checking for null is tedious, however unless you know a variable can't be null, there still a chance it will be null. There are @NotNull annotations available for FindBugs and IntelliJ, which can help you detect null values where they shouldn't be without extra coding. Optional can also play a role now.

Say you have:

   if (a != null && a.getB() != null && a.getB().getC() != null) {
        a.getB().getC().doSomething();
   }

Instead of checking for null, you can write:

    Optional.ofNullable(a)
            .map(A::getB)
            .map(B::getC)
            .ifPresent(C::doSomething);

See the answer for “ Avoid null statements” for more information.

Other useful hints:

• Converting an InputStream to a String
• How to generate numbers in a specific range
• When to use a LinkedList instead of an ArrayList
• Difference between public, default, protected and private
• How to test if an Array contains a value
• Why you should implement Runnable rather than extend Thread
• Does finally always execute?
• How to convert a String to an Enum
• Breaking out of nested loops

How to effectively iterator over a map – Note that in Java 8 you can use:

  map.forEach((k, v) -> { });

For more posts about Java, see my blog Vanilla Java. You can also contact me on Twitter @PeterLawrey.