Notes and notes about Java NULL pointer exceptions

Java. lang. NullPointerException object is used without being instantiated.

String a = new String ();

String assigns the address to.

String a = "123"

A in the constant pool

 

Because the obtained object instance does not exist, the returned address is

If you want to compare the return value of a method with a constant and put the constant before it, you can avoid calling

 

Here