When a variable is declared, it is given a type.
String color = "green";In this case, the variable color is declared to have the type String.
After this declaration, color cannot be assigned to a value that is not a String.
// A number is not a String!
String color = 8;This applies to all situations where a variable might be given a value, including delayed assignment and reassignment.
One mental model is that types are like shapes. If the type of something is a circle, you can only put circles into it.
◯ thing = ◯;You cannot put square pegs in that round hole.
// If Java actually functioned in terms of shapes, this
// would not work since a Square is not the same "type"
// of thing as a Circle.
◯ thing = ▢;