Why don't I have to import some classes? [duplicate]

Answer 1

Those classes that you don't have to import are those classes in the java.lang package.

The JLS, Chapter 7, states:

A compilation unit automatically has access to all types declared in its package and also automatically imports all of the public types declared in the predefined package java.lang.

String, Integer, Character, Exception, and Boolean are all in the java.lang package. However, ArrayList and HashMap are in the java.util package and, as you already know, need to be imported (or they could be fully qualified) if used.

Answer 2

The obvious answer is because String is used far more often than HashMap.

No. String belongs to java.lang package, which is automatically imported by the compiler.

From java.lang javadoc:

Provides classes that are fundamental to the design of the Java programming language. The most important classes are Object, which is the root of the class hierarchy, and Class, instances of which represent classes at run time.

And from Java Language Specification. Chapter 7. Packages

A package consists of a number of compilation units (§7.3). A compilation unit automatically has access to all types declared in its package and also automatically imports all of the public types declared in the predefined package java.lang.