泛型

元组件，写几个DTO

// onjava/Tuple2.java
package onjava;

public class Tuple2<A, B> {
  public final A a1;
  public final B a2;
  public Tuple2(A a, B b) { a1 = a; a2 = b; }
  public String rep() { return a1 + ", " + a2; }
  @Override
  public String toString() {
    return "(" + rep() + ")";
  }
}

这里根本不需要get和private，因为final已经保证了不可变

// onjava/Tuple3.java
package onjava;

public class Tuple3<A, B, C> extends Tuple2<A, B> {
  public final C a3;
  public Tuple3(A a, B b, C c) {
    super(a, b);
    a3 = c;
  }
  @Override
  public String rep() {
    return super.rep() + ", " + a3;
  }
}


// onjava/Tuple4.java
package onjava;

public class Tuple4<A, B, C, D>
  extends Tuple3<A, B, C> {
  public final D a4;
  public Tuple4(A a, B b, C c, D d) {
    super(a, b, c);
    a4 = d;
  }
  @Override
  public String rep() {
    return super.rep() + ", " + a4;
  }
}


// onjava/Tuple5.java
package onjava;

public class Tuple5<A, B, C, D, E>
  extends Tuple4<A, B, C, D> {
  public final E a5;
  public Tuple5(A a, B b, C c, D d, E e) {
    super(a, b, c, d);
    a5 = e;
  }
  @Override
  public String rep() {
    return super.rep() + ", " + a5;
  }
}

泛型Node的栈

// generics/LinkedStack.java
// 用链式结构实现的堆栈

public class LinkedStack<T> {
  private static class Node<U> {
    U item;
    Node<U> next;

    Node() { item = null; next = null; }
    Node(U item, Node<U> next) {
      this.item = item;
      this.next = next;
    }

    boolean end() {
      return item == null && next == null;
    }
  }

  private Node<T> top = new Node<>();  // 栈顶

  public void push(T item) {
    top = new Node<>(item, top);
  }

  public T pop() {
    T result = top.item;
    if (!top.end()) {
      top = top.next;
    }
    return result;
  }

  public static void main(String[] args) {
    LinkedStack<String> lss = new LinkedStack<>();
    for (String s : "Phasers on stun!".split(" ")) {
      lss.push(s);
    }
    String s;
    while ((s = lss.pop()) != null) {
      System.out.println(s);
    }
  }
}

RandomList

// generics/RandomList.java
import java.util.*;
import java.util.stream.*;

public class RandomList<T> extends ArrayList<T> {
  private Random rand = new Random(47);

  public T select() {
    return get(rand.nextInt(size()));
  }

  public static void main(String[] args) {
    RandomList<String> rs = new RandomList<>();
    Array.stream("The quick brown fox jumped over the lazy brown dog".split(" ")).forEach(rs::add);
    IntStream.range(0, 11).forEach(i -> 
      System.out.print(rs.select() + " "));
    );
  }
}

输出

brown over fox quick quick dog brown The brown lazy brown

一个实现Supplier<T>接口的例子

// generics/Fibonacci.java
// Generate a Fibonacci sequence
import java.util.function.*;
import java.util.stream.*;

public class Fibonacci implements Supplier<Integer> {
  private int count = 0;
  @Override
  public Integer get() { return fib(count++); }

  private int fib(int n) {
    if(n < 2) return 1;
    return fib(n-2) + fib(n-1);
  }

  public static void main(String[] args) {
    Stream.generate(new Fibonacci())
          .limit(18)
          .map(n -> n + " ")
          .forEach(System.out::print);
  }
}

我们不能直接对这么底层的代码调用，用一个适配器

// generics/IterableFibonacci.java
// Adapt the Fibonacci class to make it Iterable
import java.util.*;

public class IterableFibonacci
extends Fibonacci implements Iterable<Integer> {
  private int n;
  public IterableFibonacci(int count) { n = count; }

  @Override
  public Iterator<Integer> iterator() {
    return new Iterator<Integer>() {
      @Override
      public boolean hasNext() { return n > 0; }
      @Override
      public Integer next() {
        n--;
        return IterableFibonacci.this.get();
      }
      @Override
      public void remove() { // Not implemented
        throw new UnsupportedOperationException();
      }
    };
  }

  public static void main(String[] args) {
    for(int i : new IterableFibonacci(18))
      System.out.print(i + " ");
  }
}

因为我们用foreach就说明用了迭代，我们需要手动控制一下迭代方式

泛型方法

如果方法是 static 的，则无法访问该类的泛型类型参数，因此，如果使用了泛型类型参数，则它必须是泛型方法。

要定义泛型方法，请将泛型参数列表放置在返回值之前，如下所示：

// generics/GenericMethods.java

public class GenericMethods {
    public <T> void f(T x) {
        System.out.println(x.getClass().getName());
    }

    public static void main(String[] args) {
        GenericMethods gm = new GenericMethods();
        gm.f("");
        gm.f(1);
        gm.f(1.0);
        gm.f(1.0F);
        gm.f('c');
        gm.f(gm);
    }
}
/* Output:
java.lang.String
java.lang.Integer
java.lang.Double
java.lang.Float
java.lang.Character
GenericMethods
*/

变长参数和泛型方法

// generics/GenericVarargs.java

import java.util.ArrayList;
import java.util.List;

public class GenericVarargs {
    @SafeVarargs
    public static <T> List<T> makeList(T... args) {
        List<T> result = new ArrayList<>();
        for (T item : args)
            result.add(item);
        return result;
    }

    public static void main(String[] args) {
        List<String> ls = makeList("A");
        System.out.println(ls);
        ls = makeList("A", "B", "C");
        System.out.println(ls);
        ls = makeList(
                "ABCDEFFHIJKLMNOPQRSTUVWXYZ".split(""));
        System.out.println(ls);
    }
}
/* Output:
[A]
[A, B, C]
[A, B, C, D, E, F, F, H, I, J, K, L, M, N, O, P, Q, R,
S, T, U, V, W, X, Y, Z]
*/

这里有个小细节，我们用static泛型方法的时候,如果下面这样

public class GenericVarargs<T> {
    @SafeVarargs
    public static List<T> makeList(T... args) {
        List<T> result = new ArrayList<>();
        for (T item : args)
            result.add(item);
        return result;
    }
}
【错误，因为static方法独立于类的初始化存在，所以在类上的泛型定义是不起作用的】

public class GenericVarargs<T> {
    @SafeVarargs
    public List<T> makeList(T... args) {
        List<T> result = new ArrayList<>();
        for (T item : args)
            result.add(item);
        return result;
    }
【ok】

 一个泛型化的Supplier

// onjava/BasicSupplier.java
// Supplier from a class with a no-arg constructor
package onjava;

import java.util.function.Supplier;

public class BasicSupplier<T> implements Supplier<T> {
    private Class<T> type;

    public BasicSupplier(Class<T> type) {
        this.type = type;
    }

    @Override
    public T get() {
        try {
            // Assumes type is a public class:
            return type.newInstance();
        } catch (InstantiationException |
                IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    // Produce a default Supplier from a type token:
    public static <T> Supplier<T> create(Class<T> type) {
        return new BasicSupplier<>(type);
    }
}

// generics/CountedObject.java

public class CountedObject {
    private static long counter = 0;
    private final long id = counter++;

    public long id() {
        return id;
    }

    @Override
    public String toString() {
        return "CountedObject " + id;
    }
}

  // generics/BasicSupplierDemo.java

import onjava.BasicSupplier;

import java.util.stream.Stream;

public class BasicSupplierDemo {
    public static void main(String[] args) {
        Stream.generate(
                BasicSupplier.create(CountedObject.class))
                .limit(5)
                .forEach(System.out::println);
    }
}
/* Output:
CountedObject 0
CountedObject 1
CountedObject 2
CountedObject 3
CountedObject 4
*/

 Sets

// onjava/Sets.java

package onjava;

import java.util.HashSet;
import java.util.Set;

public class Sets {
    public static <T> Set<T> union(Set<T> a, Set<T> b) {
        Set<T> result = new HashSet<>(a);
        result.addAll(b);
        return result;
    }

    public static <T>
    Set<T> intersection(Set<T> a, Set<T> b) {
        Set<T> result = new HashSet<>(a);
        result.retainAll(b);
        return result;
    }

    // Subtract subset from superset:
    public static <T> Set<T>
    difference(Set<T> superset, Set<T> subset) {
        Set<T> result = new HashSet<>(superset);
        result.removeAll(subset);
        return result;
    }

    // Reflexive--everything not in the intersection:
    public static <T> Set<T> complement(Set<T> a, Set<T> b) {
        return difference(union(a, b), intersection(a, b));
    }
}

前三个方法通过将第一个参数的引用复制到新的 HashSet 对象中来复制第一个参数，因此不会直接修改参数集合。因此，返回值是一个新的 Set 对象。

这四种方法代表数学集合操作： union() 返回一个包含两个参数并集的 Set ， intersection() 返回一个包含两个参数集合交集的 Set ， difference() 从 superset 中减去 subset 的元素 ，而 complement() 返回所有不在交集中的元素的 Set。