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A Glossary of Name Reuse

From <<Java Puzzlers>>, Chapter 8. Classier Puzzlers


A Glossary of Name Reuse

Most of the puzzles in this chapter were based on name reuse. This section summarizes the various forms of name reuse.

Overriding

An instance method overrides all accessible instance methods with the same signature in superclasses [JLS 8.4.8.1], enabling dynamic dispatch; in other words, the VM chooses which overriding to invoke based on an instance's run-time type [JLS 15.12.4.4]. Overriding is fundamental to object-oriented programming and is the only form of name reuse that is not generally discouraged:

class Base {
public void f() { }
}
class Derived extends Base {
public void f() { } // overrrides Base.f()
}

Hiding

A field, static method, or member type hides all accessible fields, static methods, or member types, respectively, with the same name (or, for methods, signature) in supertypes. Hiding a member prevents it from being inherited [JLS 8.3, 8.4.8.2, 8.5]:

class Base {
public static void f() { }
}
class Derived extends Base {
public static void f() { } // hides Base.f()
}

Overloading

Methods in a class overload one another if they have the same name and different signatures. The overloaded method designated by an invocation is selected at compile time [JLS 8.4.9, 15.12.2]:

class CircuitBreaker {
public void f(int i)    { } // int overloading
public void f(String s) { } // String overloading
}

Shadowing

A variable, method, or type shadows all variables, methods, or types, respectively, with the same name in a textually enclosing scope. If an entity is shadowed, you cannot refer to it by its simple name; depending on the entity, you cannot refer to it at all [JLS 6.3.1]:

class WhoKnows {
static String sentence = "I don't know.";
public static void main(String[] args) {
String sentence = "I know!";   // shadows static field
System.out.println(sentence);  // prints local variable
}
}

Although shadowing is generally discouraged, one common idiom does involve shadowing. Constructors often reuse a field name from their class as a parameter name to pass the value of the named field. This idiom is not without risk, but most Java programmers have decided that the stylistic benefits outweigh the risks:

class Belt {
private final int size;
public Belt(int size) { // Parameter shadows Belt.size
this.size = size;
}
}

Obscuring

A variable obscures a type with the same name if both are in scope: If the name is used where variables and types are permitted, it refers to the variable. Similarly, a variable or a type can obscure a package. Obscuring is the only kind of name reuse where the two names are in different namespaces: variables, packages, methods, or types. If a type or a package is obscured, you cannot refer to it by its simple name except in a context where the syntax allows only a name from its namespace. Adhering to the naming conventions largely eliminates obscuring [JLS 6.3.2, 6.5]:

public class Obscure {
static String System; // Obscures type java.lang.System
public static void main(String[] args) {
// Next line won't compile: System refers to static field
System.out.println("hello, obscure world!");
}
}

posted on 2007-07-25 15:26 Scott@JAVA 阅读(324) 评论(0)  编辑  收藏 所属分类: Jave SE 6


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