Table of contents

This item list come for Effective Java Second Edition

Creating and Destroying Objects

Item 1: Consider static factory methods instead of constructors

Item 2: Consider a builder when faced with many constructor parameters

Item 3: Enforce the singleton property with a private constructor or an enum type

Item 4: Enforce noninstantiability with a private constructor

Item 5: Avoid creating unnecessary objects

Item 6: Eliminate obsolete object references

Item 7: Avoid finalizers

Methods Common to All Objects

Item 8: Obey the general contract when overriding equals

Item 9: Always override hashCode when you override equals

Item 10: Always override toString

Item 11: Override clone judiciously

Item 12: Consider implementing Comparable

Classes and Interfaces

Item 13: Minimize the accessibility of classes and members

Item 14: In public classes, use accessor methods, not public fields

Item 15: Minimize mutability

Item 16: Favor composition over inheritance

Item 17: Design and document for inheritance or else prohibit it

Item 18: Prefer interfaces to abstract classes

Item 19: Use interfaces only to define types

Item 20: Prefer class hierarchies to tagged classes

Item 21: Use function objects to represent strategies

Item 22: Favor static member classes over nonstatic

除非member classes有存取到outter class的nonstatic method,不然,應該把member classes宣告為statics


Item 23: Don’t use raw types in new code

Item 24: Eliminate unchecked warnings

Item 25: Prefer lists to arrays

Item 26: Favor generic types

Item 27: Favor generic methods

    // Generic method
    public static <E>Set<E>union(Set<E>s1, Set<E>s2) {
        Set<E>result = new HashSet<E>(s1);
        return result;

Item 28: Use bounded wildcards to increase API flexibility

Item 29: Consider typesafe heterogeneous containers


    // Typesafe heterogeneous container pattern - API
    public class Favorites {
    public <T> void putFavorite(Class<T> type, T instance);
    public <T> T getFavorite(Class<T> type);

Enums and Annotations

Item 30: Use enums instead of int constants

Item 31: Use instance fields instead of ordinals

Item 32: Use EnumSet instead of bit fields


Item 33: Use EnumMap instead of ordinal indexing


Item 34: Emulate extensible enums with interfaces


    // Emulated extensible enum using an interface
    public interface Operation {
        double apply(double x, double y);
    public enum BasicOperation implements Operation {
        PLUS("+") {
            public double apply(double x, double y) {
                return x + y;
        MINUS("-") {
            public double apply(double x, double y) {
                return x - y;
        TIMES("*") {
            public double apply(double x, double y) {
                return x * y;
        DIVIDE("/") {
            public double apply(double x, double y) {
                return x / y;
            private final String  symbol;
            BasicOperation(String symbol) {
                this.symbol = symbol;
            public String toString() {
                return symbol;
    // Emulated extension enum
    public enum ExtendedOperation implements Operation {
        EXP("^") {
            public double apply(double x, double y) {
                return Math.pow(x, y);
        REMAINDER("%") {
            public double apply(double x, double y) {
                return x % y;
        private final String  symbol;
        ExtendedOperation(String symbol) {
            this.symbol = symbol;
        public String toString() {
            return symbol;

Item 35: Prefer annotations to naming patterns

只要想到要用method name的rule做判斷邏輯時,可以考慮用annotations或許會更彈性

Item 36: Consistently use the Override annotation

Item 37: Use marker interfaces to define types


Item 38: Check parameters for validity

Item 39: Make defensive copies when needed

Item 40: Design method signatures carefully

Item 41: Use overloading judiciously

Item 42: Use varargs judiciously

Item 43: Return empty arrays or collections, not nulls

Item 44: Write doc comments for all exposed API elements

General Programming

Item 45: Minimize the scope of local variables

Item 46: Prefer for-each loops to traditional for loops

Item 47: Know and use the libraries

Item 48: Avoid float and double if exact answers are required

Item 49: Prefer primitive types to boxed primitives

Item 50: Avoid strings where other types are more appropriate

Item 51: Beware the performance of string concatenation

Item 52: Refer to objects by their interfaces

Item 53: Prefer interfaces to reflection

Item 54: Use native methods judiciously

Item 55: Optimize judiciously

Item 56: Adhere to generally accepted naming conventions


Item 57: Use exceptions only for exceptional conditions

Item 58: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors

checked exceptions應該是來被rescue的,只有能被復原的狀況,才用check exception

Item 59: Avoid unnecessary use of checked exceptions

Item 60: Favor the use of standard exceptions

Item 61: Throw exceptions appropriate to the abstraction

Item 62: Document all exceptions thrown by each method

Item 63: Include failure-capture information in detail messages

Item 64: Strive for failure atomicity

Item 65: Don’t ignore exceptions


Item 66: Synchronize access to shared mutable data

Item 67: Avoid excessive synchronization

Item 68: Prefer executors and tasks to threads

Item 69: Prefer concurrency utilities to wait and notify

Item 70: Document thread safety

Item 71: Use lazy initialization judiciously

Item 72: Don’t depend on the thread scheduler

Item 73: Avoid thread groups


Item 74: Implement Serializable judiciously

Item 75: Consider using a custom serialized form

Item 76: Write readObject methods defensively

Item 77: For instance control, prefer enum types to readResolve

Item 78: Consider serialization proxies instead of serialized instances