They are null by default for objects, 0 for numeric values and false for booleans. For variables declared in methods - Java requires them to be initialized. Not initializing them causes a compile time error when they are accessed.

Are uninitialized variables null in Java?

Java does not have uninitialized variables. ... Python initializes local variables to NULL (distinct from None ) and raises an UnboundLocalError when such a variable is accessed before being (re)initialized to a valid value.

What does it mean when a variable is uninitialized?

An uninitialized variable has an undefined value, often corresponding to the data that was already in the particular memory location that the variable is using. This can lead to errors that are very hard to detect since the variable's value is effectively random, different values cause different errors or none at all.