Java: Why am I required to initialize a primitive local variable?

Question

public class Foo {     public static void main(String[] args) {         float f;         System.out.println(f);     } } 

The print statement causes the following compile-time error,

The local variable f may not have been initialized

If primitives in Java already have a default value (float = 0.0f), why am I required to define one?

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Edit:

So, this works

public class Foo {     float f;     public static void main(String[] args) {         System.out.println(new Foo().f);     } } 

Thanks, everyone!

Answer 1

Because it's a local variable. This is why nothing is assigned to it :

Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.

Edit: Why does Java raise this compilation error ? If we look at the IdentifierExpression.java class file, we will find this block :

... if (field.isLocal()) {             LocalMember local = (LocalMember)field;             if (local.scopeNumber < ctx.frameNumber && !local.isFinal()) {                 env.error(where, "invalid.uplevel", id);             }             if (!vset.testVar(local.number)) {                 env.error(where, "var.not.initialized", id);                 vset.addVar(local.number);             }             local.readcount++;         } ... 

As stated (if (!vset.testVar(local.number)) {), the JDK checks (with testVar) if the variable is assigned (Vset's source code where we can find testVar code). If not, it raises the error var.not.initialized from a properties file :

... javac.err.var.not.initialized=\     Variable {0} may not have been initialized. ... 

Source

Answer 2

In fact, the compiler does not assign a default value to your float f, because in this case it is a local variable -- and not a field:

Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.