第17章 容器深入研究

17.1 完整的容器分类法

17.2 填充容器

这一小节，简单的介绍了java.util.Collections中的两个方法：

public static <T> List<T> nCopies(int n,T o)

public static <T> void fill(List<? super T> list,T obj)

这两个方法可以多次拷贝同一个对象到一个列表中

java代码：

//The Collections.fill() & Collections.nCopies() methods.package containers;import java.util.ArrayList;import java.util.Collections;import java.util.List;class StringAddress {private String s;public StringAddress(String s) {this.s = s;}@Overridepublic String toString() {return super.toString() + " " + s;}}public class FillingLists {public static void main(String[] args) {List<StringAddress> list = new ArrayList<StringAddress>(Collections.nCopies(4, new StringAddress("Hello")));System.out.println(list);Collections.fill(list, new StringAddress("World"));System.out.println(list);}}

运行结果：

[containers.StringAddress@4e81d783 Hello, containers.StringAddress@4e81d783 Hello, containers.StringAddress@4e81d783 Hello, containers.StringAddress@4e81d783 Hello][containers.StringAddress@2e6c01b9 World, containers.StringAddress@2e6c01b9 World, containers.StringAddress@2e6c01b9 World, containers.StringAddress@2e6c01b9 World]

A Generator solution

package net.mindview.util;import java.util.ArrayList;/** * 该类本身就是一个ArrayList，把Generator生成器生成的对象添加到该类中 * list方法返回一个CollectionData对象 */public class CollectionData<T> extends ArrayList<T> {public CollectionData(Generator<T> gen, int quantity) {for (int i = 0; i < quantity; i++) {add(gen.next());}}// A generic convenience method:public static <T> CollectionData<T> list(Generator<T> gen, int quantity) {return new CollectionData<T>(gen, quantity);}}

把生成器对象转换称Collection

测试代码：

package containers;import java.util.LinkedHashSet;import java.util.Set;import net.mindview.util.CollectionData;import net.mindview.util.Generator;class Government implements Generator<String>{String[] foundation = ("strange women lying in ponds " +"distributing swords is no basis for a system of " +"government").split(" ");private int index;@Overridepublic String next() {return foundation[index++];}}public class CollectionDataTest {public static void main(String[] args) {Set<String> set = new LinkedHashSet<>(new CollectionData<>(new Government(), 15));//Using the convenience methodset.addAll(CollectionData.list(new Government(), 15));System.out.println(set);}}

package containers;import java.util.ArrayList;import java.util.HashSet;import net.mindview.util.CollectionData;import net.mindview.util.RandomGenerator;public class CollectionDataGeneration {public static void main(String[] args) {System.out.println(new ArrayList<String>(CollectionData.list(//Convenience methodnew RandomGenerator.String(9),10)));System.out.println(new HashSet<Integer>(new CollectionData<Integer>(new RandomGenerator.Integer(),10)));}}

在实验的过程中需要用的其他类：

//: net/mindview/util/Generator.java// A generic interface.package net.mindview.util;public interface Generator<T> { T next(); } ///:~

//: net/mindview/util/CountingGenerator.java// Simple generator implementations.package net.mindview.util;public class CountingGenerator {  public static class  Boolean implements Generator<java.lang.Boolean> {    private boolean value = false;    public java.lang.Boolean next() {      value = !value; // Just flips back and forth      return value;    }  }  public static class  Byte implements Generator<java.lang.Byte> {    private byte value = 0;    public java.lang.Byte next() { return value++; }  }  static char[] chars = ("abcdefghijklmnopqrstuvwxyz" +    "ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();  public static class  Character implements Generator<java.lang.Character> {    int index = -1;    public java.lang.Character next() {      index = (index + 1) % chars.length;      return chars[index];    }  }  public static class  String implements Generator<java.lang.String> {    private int length = 7;    Generator<java.lang.Character> cg = new Character();    public String() {}    public String(int length) { this.length = length; }    public java.lang.String next() {      char[] buf = new char[length];      for(int i = 0; i < length; i++)        buf[i] = cg.next();      return new java.lang.String(buf);    }  }  public static class  Short implements Generator<java.lang.Short> {    private short value = 0;    public java.lang.Short next() { return value++; }  }  public static class  Integer implements Generator<java.lang.Integer> {    private int value = 0;    public java.lang.Integer next() { return value++; }  }  public static class  Long implements Generator<java.lang.Long> {    private long value = 0;    public java.lang.Long next() { return value++; }  }  public static class  Float implements Generator<java.lang.Float> {    private float value = 0;    public java.lang.Float next() {      float result = value;      value += 1.0;      return result;    }  }  public static class  Double implements Generator<java.lang.Double> {    private double value = 0.0;    public java.lang.Double next() {      double result = value;      value += 1.0;      return result;    }  }} ///:~

//: net/mindview/util/RandomGenerator.java// Generators that produce random values.package net.mindview.util;import java.util.*;public class RandomGenerator {  private static Random r = new Random(47);  public static class  Boolean implements Generator<java.lang.Boolean> {    public java.lang.Boolean next() {      return r.nextBoolean();    }  }  public static class  Byte implements Generator<java.lang.Byte> {    public java.lang.Byte next() {      return (byte)r.nextInt();    }  }  public static class  Character implements Generator<java.lang.Character> {    public java.lang.Character next() {      return CountingGenerator.chars[        r.nextInt(CountingGenerator.chars.length)];    }  }  public static class  String extends CountingGenerator.String {    // Plug in the random Character generator:    { cg = new Character(); } // Instance initializer    public String() {}    public String(int length) { super(length); }  }  public static class  Short implements Generator<java.lang.Short> {    public java.lang.Short next() {      return (short)r.nextInt();    }  }  public static class  Integer implements Generator<java.lang.Integer> {    private int mod = 10000;    public Integer() {}    public Integer(int modulo) { mod = modulo; }    public java.lang.Integer next() {      return r.nextInt(mod);    }  }  public static class  Long implements Generator<java.lang.Long> {    private int mod = 10000;    public Long() {}    public Long(int modulo) { mod = modulo; }    public java.lang.Long next() {      return new java.lang.Long(r.nextInt(mod));    }  }  public static class  Float implements Generator<java.lang.Float> {    public java.lang.Float next() {      // Trim all but the first two decimal places:      int trimmed = Math.round(r.nextFloat() * 100);      return ((float)trimmed) / 100;    }  }  public static class  Double implements Generator<java.lang.Double> {    public java.lang.Double next() {      long trimmed = Math.round(r.nextDouble() * 100);      return ((double)trimmed) / 100;    }  }} ///:~

Map generators