Name three rules that needs to be used when declaring variables in java language

Variable Names: In Java, variable names are case-sensitive and they must begin with a letter. They can also begin with a dollar sign ($) or an underscore (_), but it's not recommended as they are typically used in special cases. The subsequent characters may be letters, digits, dollar signs, or underscore characters.

No Reserved Words: You cannot use reserved words in Java as variable names. For example, words like "int", "void", "class", "public", etc., are reserved words and they have special meaning in Java. Using them as variable names would cause a compile-time error.

Initialization: In Java, local variables are not given a default value. You must initialize local variables before you can use them. If you do not initialize a local variable and try to use it, the compiler will give you an error. However, instance and class variables are given a default value if you do not initialize them. For example, for numbers the default value is 0, for booleans it's false, and for object references it's null.