<rt id="bn8ez"></rt>
<label id="bn8ez"></label>

  • <span id="bn8ez"></span>

    <label id="bn8ez"><meter id="bn8ez"></meter></label>

    Vincent.Chan‘s Blog

    常用鏈接

    統計

    積分與排名

    網站

    最新評論

    《Effective Java》Chapter 6

    Chapter 6. Methods

    Item 23: Check parameters for validity
    Each time you write a method or constructor, you should think about what restrictions exist on its parameters. You should document these restrictions and enforce them with explicit checks at the beginning of the method body.

    Item 24: Make defensive copies when needed
    You must program defensively with the assumption that clients of your class will do their best to destroy its invariants.
    // Broken "immutable" time period class
    public final class Period {
        
    private final Date start;
        
    private final Date end;
        
        
    /**
         * 
    @param start
         *            the beginning of the period.
         * 
    @param end
         *            the end of the period; must not precede start.
         * 
    @throws IllegalArgumentException
         *             if start is after end.
         * 
    @throws NullPointerException
         *             if start or end is null.
         
    */

        
    public Period(Date start, Date end) {
            
    if (start.compareTo(end) > 0)
                
    throw new IllegalArgumentException(start + " after "
                        
    + end);
            
    this.start = start;
            
    this.end = end;
        }

        
        
    public Date start() {
            
    return start;
        }

        
        
    public Date end() {
            
    return end;
        }

        
         
    // Remainder omitted
    }
    Violate this invariant by exploiting the fact that Date is mutable:
    // Attack the internals of a Period instance
    Date start = new Date();
    Date end 
    = new Date();
    Period p 
    = new Period(start, end);
    end.setYear(
    78); // Modifies internals of p!
    It is essential to make a defensive copy of each mutable parameter to the constructor:
    // Repaired constructor - makes defensive copies of parameters
    public Period(Date start, Date end) {
        
    this.start = new Date(start.getTime());
        
    this.end = new Date(end.getTime());
        
    if (this.start.compareTo(this.end) > 0)
            
    throw new IllegalArgumentException(start +" after "+ end);
    }
    Defensive copies are made before checking the validity of the parameters (Item 23), and the validity check is performed on the copies rather than on the originals.
    Do not use the clone method to make a defensive copy of a parameter whose type is subclassable by untrusted parties.

    // Second attack on the internals of a Period instance
    Date start = new Date();
    Date end 
    = new Date();
    Period p 
    = new Period(start, end);
    p.end().setYear(
    78); // Modifies internals of p!
    To defend against the second attack, merely modify the accessors to return defensive copies of mutable internal fields:
    // Repaired accessors - make defensive copies of internal fields
    public Date start() {
        
    return (Date) start.clone();
    }


    public Date end() {
        
    return (Date) end.clone();
    }
    Note that the new accessors, unlike the new constructor, do use the clone method to make defensive copies. This is acceptable (although not required), as we know with certainty that the class of Period's internal Date objects is java.util.Date rather than some potentially untrusted subclass.

    Item 25: Design method signatures carefully
    Too many methods make a class difficult to learn, use, document, test, and maintain. This is doubly true for interfaces.
    Avoid long parameter lists (3 as practical max).
    Long sequences of identically typed parameters are especially harmful.
    For parameter types, favor interfaces over classes.
    Use function objects judiciously.

    Item 26: Use overloading judiciously
    Selection among overloaded methods is static (at compile time), while selection among overridden methods is dynamic (at run time).

    //Broken - incorrect use of overloading!
    public class CollectionClassifier {
        
    public static String classify(Set s) {
            
    return "Set";
        }


        
    public static String classify(List l) {
            
    return "List";
        }


        
    public static String classify(Collection c) {
            
    return "Unknown Collection";
        }


        
    public static void main(String[] args) {
            Collection[] tests 
    = new Collection[] new HashSet(), // A Set
                    new ArrayList(), // A List
                    new HashMap().values() // Neither Set nor List
            }
    ;
            
    for (int i = 0; i < tests.length; i++)
                System.out.println(classify(tests[i]));
        }

    }
    For all three iterations of the loop, the compile-time type of the parameter is the same: Collection. The run-time type is different in each iteration, but this does not affect the choice of overloading.

    class A {
        String name() 
    {
            
    return "A";
        }

    }


    class B extends A {
        String name() 
    {
            
    return "B";
        }

    }


    class C extends A {
        String name() 
    {
            
    return "C";
        }

    }


    public class Overriding {
        
    public static void main(String[] args) {
            A[] tests 
    = new A[] new A(), new B(), new C() };
            
    for (int i = 0; i < tests.length; i++)
                System.out.print(tests[i].name());
        }

    }
    As you would expect, this program prints out "ABC" even though the compile-time type of the instance is A in each iteration of the loop.

    You should generally refrain from overloading methods with multiple signatures that have the same number of parameters. In some cases, especially where constructors are involved, it may be impossible to follow this advice. In that case, you should at least avoid situations where the same set of parameters can be passed to different overloadings by the addition of casts.

    Item 27: Return zero-length arrays, not nulls
    It is possible to return the same zero-length array from every invocation that returns no items because zero-length arrays are immutable and immutable objects may be shared freely (Item 13).
    private List cheesesInStock = ;

    private final static Cheese[] NULL_CHEESE_ARRAY = new Cheese[0];

    /**
     * 
    @return an array containing all of the cheeses in the shop.
     
    */

    public Cheese[] getCheeses() {
        
    return (Cheese[]) cheesesInStock.toArray(NULL_CHEESE_ARRAY);
    }

    There is no reason ever to return null from an array-valued method instead of returning a zero-length array.

    Item 28: Write doc comments for all exposed API elements
    /**
    * Returns the element at the specified position in this list.
    *
    @param index index of element to return; must be
    * nonnegative and less than the size of this list.
    @return the element at the specified position in this list.
    @throws IndexOutOfBoundsException if the index is out of range
    * (<tt>index &lt; 0 || index &gt;= this.size()</tt>).
    */

    Object get(
    int index)
    For methods and constructors, the summary description should be a verb phrase describing the action performed by the method.
    For classes, interfaces, and fields, the summary description should be a noun phrase describing the thing represented by an instance of the class or interface or by the field itself.
    Be careful not to include a period within the first sentence of a doc comment.

    posted on 2006-02-18 18:32 Vincent.Chen 閱讀(167) 評論(0)  編輯  收藏 所屬分類: Java

    主站蜘蛛池模板: 黄网站色成年片大免费高清 | 国产一精品一AV一免费孕妇| 免费人成再在线观看网站| 亚洲一区二区三区在线观看蜜桃| 亚洲中文字幕日产乱码高清app| 噜噜嘿在线视频免费观看| 大学生一级毛片免费看| 57pao一国产成永久免费| 国产乱子精品免费视观看片| 91在线视频免费看| 毛片免费观看的视频| 国产一区二区三区在线免费观看| 四虎成人免费网站在线| AV在线亚洲男人的天堂| 怡红院亚洲怡红院首页| 久久亚洲sm情趣捆绑调教| 亚洲精品天堂在线观看| 苍井空亚洲精品AA片在线播放| 免费亚洲视频在线观看| 中国好声音第二季免费播放| a级日本高清免费看| 免费电影在线观看网站| 国产一级大片免费看| 亚洲中文字幕久久精品无码喷水| 亚洲欧洲日韩不卡| 亚洲伊人久久大香线蕉AV| 尤物视频在线免费观看| 久久青草免费91线频观看不卡 | 亚洲综合激情五月色一区| 在线播放国产不卡免费视频| 国产精品免费观看| 国产成人aaa在线视频免费观看 | 亚洲综合无码精品一区二区三区| 亚洲精品无码久久久| 亚洲国产婷婷六月丁香| 亚洲国产精品美女| 另类图片亚洲校园小说区| 日本三级在线观看免费| 好先生在线观看免费播放| 亚洲精品无码AV中文字幕电影网站 | 三上悠亚在线观看免费|