Android异常--空指针异常

总结自: Java Tips and Best Practices to avoid NPE in Java Applications

1. Call equals() and equalsIgnoreCase() method on known String literal rather unknown object

Object unknownObject = null;

//wrong way - may cause NullPointerException
if(unknownObject.equals("knownObject")){
   System.err.println("This may result in NullPointerException if unknownObject is null");
}

//right way - avoid NullPointerException even if unknownObject is null
if("knownObject".equals(unknownObject)){
    System.err.println("better coding avoided NullPointerException");
}

2. Prefer valueOf() over toString() where both return same result

BigDecimal bd = getPrice();
System.out.println(String.valueOf(bd)); //doesn’t throw NPE
System.out.println(bd.toString()); //throws "Exception in thread "main" java.lang.NullPointerException"

3. Using null safe methods and libraries

//StringUtils methods are null safe, they don't throw NullPointerException
System.out.println(StringUtils.isEmpty(null));
System.out.println(StringUtils.isBlank(null));
System.out.println(StringUtils.isNumeric(null));
System.out.println(StringUtils.isAllUpperCase(null));

Output:
true
true
false
false

4. Avoid returning null from method, instead return empty collection or empty array.

public List getOrders(Customer customer){
   List result = Collections.EMPTY_LIST;
   return result;
}

5. Use of annotation @NotNull and @Nullable
6. void unnecessary autoboxing and unboxing in your code

Person ram = new Person("ram");
int phone = ram.getPhone();

7. Follow Contract and define reasonable default value
8. Use Null Object Pattern