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CMSC 132:
Object-Oriented Programming II
Effective Java
Department of Computer Science
University of Maryland, College Park
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Effective Java
Title
Effective Java Programming Language Guide
Author
Joshua Bloch
Contents
Learn to use Java language and its libraries more effectively
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Java Puzzlers (By J. Bloch)
Java
Simple and elegant Need to avoid some sharp corners!
Puzzlers
Java code fragments Expose some tricky aspects of Java
Effective Java
Patterns and idioms to emulate Pitfalls to avoid
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What's In A Name?
public class Name { private String myName; public Name(String n) { myName = n; } public boolean equals(Object o) { if (!(o instanceof Name)) return false; Name n = (Name)o; return myName.equals(n.myName); } public static void main(String[ ] args) { Set s = new HashSet(); s.add(new Name("Donald")); System.out.println( s.contains(new Name("Donald"))); } }
Output
- True
- False
- It Varies
Name class violates Java hashCode( ) contract.
If you override equals( ), must also override hashCode( )!
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You're Such A Character
public class Trivial {
public static void main(String args[ ]) { System.out.print("H" + "a"); System.out.print('H' + 'a'); }
}
Output
- Ha
- HaHa
- Neither
Prints Ha
'H' + 'a' evaluated as int , then converted to String!
Use string concatenation (+) with care. At least one operand must be a String 6
The Confusing Constructor
public class Confusing { public Confusing(Object o) { System.out.println("Object"); } public Confusing(double[] dArray) { System.out.println("double array"); } public static void main(String args[]) { new Confusing(null); } }
Output
- Object
- double array
- Neither
When multiple overloadings apply, the most specific wins
Avoid overloading. If you overload, avoid ambiguity
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Time For A Change
Problem
If you pay $2.00 for a gasket that costs $1.10, how much change do you get?
public class Change { public static void main(String args[ ]) { System.out.println(2.00 - 1.10); } }
Output
- Neither
Prints 0.8999999999999999. Decimal values can’t be represented exactly by float or double
Avoid float or double where exact answers are required. Use BigDecimal, int, or long instead 8
A Private Matter
class Base { public String name = "Base"; } class Derived extends Base { private String name = "Derived"; } public class PrivateMatter { public static void main(String[] args) { System.out.println(new Derived( ).name); } }
Output
- Derived
- Base
- Neither
Compiler error in class PrivateMatter: Can't access name
Private field can hide public. Avoid hiding & public fields
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Effective Java Topics
1. Creating and Destroying Objects
2. Methods Common to All Objects
3. Classes and Interfaces
4. Substitutes for C Constructs
5. Methods
6. General Programming
7. Exceptions
8. Threads
9. Serialization
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Creating and Destroying Objects
Consider providing static factory methods instead of constructors Enforce singleton property with a private constructor Enforce noninstantiability with a private constructor Avoid creating duplicate objects Eliminate obsolete object references Avoid finalizers
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Methods Common to All Objects
Obey the general contract when overriding equals Always override hashCode when you override equals Always override toString Override clone judiciously Consider implementing Comparable
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Classes and Interfaces
Minimize the accessibility of classes and members Favor immutability Favor composition over inheritance Design and document for inheritance or else prohibit it Prefer interfaces to abstract classes Use interfaces only to define types Favor static member classes over nonstatic
