数组

数组为什么特殊

 

package arrays;

//: arrays/ContainerComparison.java
import java.util.*;

class BerylliumSphere {
    private static long counter;
    private final long id = counter++;

    public String toString() {
        return "Sphere " + id;
    }
}

public class ContainerComparison {
    public static void main(String[] args) {
        BerylliumSphere[] spheres = new BerylliumSphere[10];
        for (int i = 0; i < 5; i++)
            spheres[i] = new BerylliumSphere();
        System.out.println(Arrays.toString(spheres));
        System.out.println(spheres[4]);

        List<BerylliumSphere> sphereList = new ArrayList<BerylliumSphere>();
        for (int i = 0; i < 5; i++)
            sphereList.add(new BerylliumSphere());
        System.out.println(sphereList);
        System.out.println(sphereList.get(4));

        int[] integers = { 0, 1, 2, 3, 4, 5 };
        System.out.println(Arrays.toString(integers));
        System.out.println(integers[4]);

        List<Integer> intList = new ArrayList<Integer>(Arrays.asList(0, 1, 2, 3, 4, 5));
        intList.add(97);
        System.out.println(intList);
        System.out.println(intList.get(4));
    }
}

输出结果：

数组是第一级对象

                      

package arrays;

//: arrays/ArrayOptions.java
//Initialization & re-assignment of arrays.
import java.util.*;

public class ArrayOptions {
    public static void main(String[] args) {
        // Arrays of objects:
        BerylliumSphere[] a; // 未初始化的局部变量，在对它正确地初始化之前，编译器不能用此引用做任何事情
        BerylliumSphere[] b = new BerylliumSphere[5];//此时并没有对象注入数组，但可以询问数组大小    。缺点：无法知道数组中有多少个确切的元素，length只表示数组能够容纳多少个元素，而不是实际保持元素个数
        // The references inside the array are
        // automatically initialized to null:
        System.out.println("b: " + Arrays.toString(b));
        BerylliumSphere[] c = new BerylliumSphere[4];//动态初始化
        for (int i = 0; i < c.length; i++)
            if (c[i] == null) // Can test for null reference
                c[i] = new BerylliumSphere();
        // 静态初始化的简化形式（聚集初始化）
        BerylliumSphere[] d = { new BerylliumSphere(), new BerylliumSphere(), new BerylliumSphere() };
        // 动态的聚集初始化，可以在任何位置创建和初始化数组对象
        a = new BerylliumSphere[] { new BerylliumSphere(), new BerylliumSphere(), };
        // (Trailing comma is optional in both cases)
        System.out.println("a.length = " + a.length);
        System.out.println("b.length = " + b.length);
        System.out.println("c.length = " + c.length);
        System.out.println("d.length = " + d.length);
        a = d;
        System.out.println("a.length = " + a.length);

        // Arrays of primitives:
        int[] e; // Null reference
        int[] f = new int[5];
        // The primitives inside the array are
        // automatically initialized to zero:
        System.out.println("f: " + Arrays.toString(f));
        int[] g = new int[4];
        for (int i = 0; i < g.length; i++)
            g[i] = i * i;
        int[] h = { 11, 47, 93 };
        // Compile error: variable e not initialized:
        // !System.out.println("e.length = " + e.length);
        System.out.println("f.length = " + f.length);
        System.out.println("g.length = " + g.length);
        System.out.println("h.length = " + h.length);
        e = h;
        System.out.println("e.length = " + e.length);
        e = new int[] { 1, 2 };
        System.out.println("e.length = " + e.length);
    }
} 

 

注：

基本类型数据的初始化

public class Test {
    public static void main(String args[]){
        //定義一個int[]类型的数组变量
        int[] iArr;
        //静态初始化数组，数组长度为3
        iArr=new int[]{2,5,-12};
    }
}

          

注：当通过引用变量来访问实例属性，或者调用非静态方法时，如果该引用变量还未引用一个有效的对象，程序就会引发NullPointerException运行时异常

 引用型数组的初始化

 返回一个数组

package arrays;

//: arrays/IceCream.java
// Returning arrays from methods.
import java.util.*;

public class IceCream {
    private static Random rand = new Random(47);
    static final String[] FLAVORS = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip",
            "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" };

    public static String[] flavorSet(int n) {
        if (n > FLAVORS.length)
            throw new IllegalArgumentException("Set too big");
        String[] results = new String[n];
        boolean[] picked = new boolean[FLAVORS.length];
        for (int i = 0; i < n; i++) {
            int t;
            do
                t = rand.nextInt(FLAVORS.length);
            while (picked[t]);
            results[i] = FLAVORS[t];
            picked[t] = true;
        }
        return results;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 7; i++)
            System.out.println(Arrays.toString(flavorSet(3)));
    }
}

 输出：

 多维数组

package arrays;

import java.util.*;

public class MultidimensionalPrimitiveArray {
    public static void main(String[] args) {
        int[][] a = { { 1, 2, 3, }, { 4, 5, 6, }, };
        System.out.println(Arrays.deepToString(a));
    }
}

输出：

package arrays;

import java.util.*;

public class ThreeDWithNew {
  public static void main(String[] args) {
    // 3-D array with fixed length:
    int[][][] a = new int[2][2][4];
    System.out.println(Arrays.deepToString(a));
  }
}

输出：

package arrays;

import java.util.*;

public class RaggedArray {
    public static void main(String[] args) {
        Random rand = new Random(47);
        // 3-D array with varied-length vectors:
        int[][][] a = new int[rand.nextInt(7)][][];
        for (int i = 0; i < a.length; i++) {
            a[i] = new int[rand.nextInt(5)][];
            for (int j = 0; j < a[i].length; j++)
                a[i][j] = new int[rand.nextInt(5)];
        }
        System.out.println(Arrays.deepToString(a));
    }
}

import java.util.*;

public class MultidimensionalObjectArrays {
  public static void main(String[] args) {
    BerylliumSphere[][] spheres = {
      { new BerylliumSphere(), new BerylliumSphere() },
      { new BerylliumSphere(), new BerylliumSphere(),
        new BerylliumSphere(), new BerylliumSphere() },
      { new BerylliumSphere(), new BerylliumSphere(),
        new BerylliumSphere(), new BerylliumSphere(),
        new BerylliumSphere(), new BerylliumSphere(),
        new BerylliumSphere(), new BerylliumSphere() },
    };
    System.out.println(Arrays.deepToString(spheres));
  }
} /* Output:
[[Sphere 0, Sphere 1], [Sphere 2, Sphere 3, Sphere 4, Sphere 5], [Sphere 6, Sphere 7, Sphere 8, Sphere 9, Sphere 10, Sphere 11, Sphere 12, Sphere 13]]
*///:~

import java.util.*;

public class AutoboxingArrays {
  public static void main(String[] args) {
    Integer[][] a = { // Autoboxing:
      { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 },
      { 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 },
      { 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 },
      { 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 },
    };
    System.out.println(Arrays.deepToString(a));
  }
} /* Output:
[[1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [21, 22, 23, 24, 25, 26, 27, 28, 29, 30], [51, 52, 53, 54, 55, 56, 57, 58, 59, 60], [71, 72, 73, 74, 75, 76, 77, 78, 79, 80]]
*///:~