Avoid NullPointerException in Java

I would like to share with you some tips to deal/prevent NPE in your code:

The BIG IMPORTANT rule: do NOT assign / pass / return null value and keep your code as cohesive as possible.

Programming tips / techniques:
1. Return an empty collections instead of null.
For example:
List:

return Collections.emptyList()

Set:

return Collections.emptySet()

Map:

return Collections.emptyMap()

org.apache.commons.lang.ArrayUtils.EMPTY_XXX_ARRAY

2. Return

org.apache.commons.lang.StringUtils.EMPTY

or “” instead of null

3. Return “UNKNOWN” enum instead of null

4. Check null before executing a variable

5. Null-safe checking technique

if("Hello".equals(hello)) { }

instead of

if(hello != null) {
	if(hello.equals("Hello")) { }
}

or

if(hello.equals("Hello")) { }

Principle and Pattern:
6. Null Object Pattern (https://sourcemaking.com/design_patterns/null_object/java-0)
(My favourite pattern belong with Template Method and Bridge)
Instead of

public Manager getManager(ManagerType inType){
	switch(inType) {
		case PROJECT:
			return ProjectManager();
		case DEPARTMENT:
			return DepartmentManager();
		default:
			break;
	}
	return null;
}

Let try:

public Manager getManager(ManagerType inType){
	switch(inType) {
		case PROJECT:
			return ProjectManager();
		case DEPARTMENT:
			return DepartmentManager();
		default:
		break;
	}
	return NullManager();
}

Design tips:

class NullManager extends Manager {
	// Implement all abstract methods of Manager
	// Override neccessary methods to drive to do "Nothing" or "Default" action.
}

7. Avoid too many dot syntax. (http://en.wikipedia.org/wiki/Law_of_Demeter)
For example:

getSettingManager().getSettingPanel().getHeaderPanel().getHeaderLabel().setText("Setting");

Class Variable:
8. Try initializing variable member of class before using it.
Direct initialization

private List mPersons = new ArrayList();

Initialize them in class Constructor

public Family() {
	mPersons = new ArrayList();
}

Constructor dependency injection

public Family(List inPersons) {
	mPersons = inPersons;
}

Provide accessibility through getter and using lazy initialization

public getPersons() {
	if(mPersons == null) {
	mPersons = new ArrayList();
	}
	return mPersons;
}

9. Avoid / eliminate method dependency injection (setter)

public setPersons(List inPersons) {
	mPersons = inPersons;
}

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