Nulls and Validity Checks

Avoid NullPointerExceptions (NPE)

• Check at the beginning - Unhappy path, Happy path

• Consider using helper methods

• Check each object which is de-referenced in the method - and only these ones

Example: Bad: Null check at the wrong place

void doSomething(MyClass input) {
    if(input != null) { // Unnecessary here
        doStuff(input);
    }
}

void doStuff(MyClass input) {
    input.setData(10); // Would be necessary here
}

Example: Good: Null check always and only when dereferencing

void doSomething(MyClass input) {
    doStuff(input);
}

void doStuff(MyClass input) {
    if(input == null) {
        return;
    }
    input.setData(10);
}

Defensive Programming

Also known as Secure Programming. A larger topic, but some elements:

Inputs

• Never trust data coming from outside - user input, external systems

• Fail early, Fail clean

• Create and use immutable types

Outputs

• Do not modify a data through getters

• Prevent it if possible - unmodifiable collections

• Create and use immutable types

Example: Bad: Unclear validity check

// What to do when input source is null?
// Why is the entire content indented?
public final void parse(final Config[] configs, final boolean flushAfterParse) {
    if (getInputSource() != null) {
        final EdifactParser parser = new EdifactParser(getInputSource(), getHandler());
        for (final Config config : configs) {
            parser.addConfig(config);
        }
        parser.convert();
        if (flushAfterParse) {
            getHandler().flush(true);
        }
    }
}

Example: Good: Validity check at the beginning

public final void parse(final Config[] configs, final boolean flushAfterParse) {
    // Unhappy path
    if (getInputSource() == null) {
        return; // Do nothing, return default or throw exception
    }
    // Happy path or Normal flow
    final EdifactParser parser = new EdifactParser(getInputSource(), getHandler());
    for (final Config config : configs) {
        parser.addConfig(config);
    }
    parser.convert();
    if (flushAfterParse) {
        getHandler().flush(true);
    }
}

Parameters

• Do not pass explicitly null - it is a "cheating", _allow it with an overloaded method

Unit Test of Inputs

• Unit test will nulls and empty values in all possible ways

  • null object

  • entire collection or array is null

  • collection or array element is null

Error Handling

• Do not return null - rather throw exception

Things must not be multiplied beyond necessity. The simplest solution is the best. (Occam’s Razor)