An identifier is a unlimited-length sequence of Unicode letters and digits, the first of which must be a letter.
The method Character.isJavaIdentifierStart returns true when passed a Unicode character that is considered to be a letter in a identifier. The method Character.isJavaIdentifierPart returns true when passed a Unicode character that is considered to be a a letter or a digit in an indentifier.
An identifier must not be the same as a boolean literal, the null literal, or a keyword in the Java programming language. (查看原文)
A literal is the source code representation of a value a primitive type, the String type, or the null type. String literals and, more generally, strings that are the value of constant expressions are "interned" so as to unique instances, using the method String.intern.
The null type has one value, the null reference, denoted by the literal null. The boolean type has two values, denoted by the literals true and false. (查看原文)
The Java programming language is strongly typed, which means that every variable and every expression has a type that is known at compile time.
The type of the Java Programming language are divided into two categories: primitive types and reference types, there is a special null type, the type of expression null, which has no name.The null type is only possible value of an expression of null type and can always converted to any reference type.
Corresponding to the primitive types and reference types, there are two categories of data values that can be stored in variables, passed as arguments, returned by methods, and operated upon: primitive values and reference values. (查看原文)
