When JVM encounters a data type of lower size which occupies less memory, it is assigned to a data type of higher size implicitly by the JVM. This is also known as automatic type conversion. For Example

int i = 1; // 4 bytes
double d = i; // 8 bytes

When a data type of higher size which occupies more memory, needs to be assigned to a data type of lower size, it is called explicit casting. This type of casting won't be done implicitly by the JVM. This casting operation should be performed by the programmer. For example 

double d = 1.0; 
int i = (int) d;