Implicit and explicit casting : Primitive Type Casting « Type Casting « SCJP

1. SCJP
2. Type Casting
3. Primitive Type Casting

An implicit cast means you don't have to write code for the cast

An implicit cast happens when you're doing a widening conversion. 

public class MainClass{
    public static void main(String[] argv){
        int a = 100;
        long b = a; // Implicit cast, an int value always fits in a long
    }
}

An explicit casts looks like this:

public class MainClass{
    public static void main(String[] argv){
        float a = 100.001f;
        int b = (int)a; // Explicit cast, the float could lose info
    }
}
    
Impossible Conversions

Any primitive type to any reference type

The null value to any primitive type

Any primitive to boolean

A boolean to any primitive