JAVA菜鸟入门(9) Java打印一维数组，二维数组

一 打印list中的元素

0 JAVA风格的最简做法

import java.util.List;
...
List<Integer> l = new ArrayList(3);
System.out.println(l);

1 JAVA风格的做法

import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
...
List<Integer> l = new ArrayList(3);
System.out.println(Arrays.toString(l.toArray()));

2  类似C++的用法 

List<Integer> l = new ArrayList(3);
for (int i = 0; i < l.size(); ++i) {
     System.out.println(l.get(i));
}

3  for each 的方法 

List<Integer> l = pt.getRow(4);
for (Integer i: l) {
     System.out.println(i);
}

二. 打印一维数组
1 JAVA风格的做法

int [] aa = new int [] {1, 2, 3, 4, 5};
System.out.println(aa.toString()); //[I@103c520 注意这个写法是错误的，打印出来的是垃圾值。
System.out.println(Arrays.toString(aa)); //[1, 2, 3, 4, 5]

2 类似 C++的for循环

for (int i = 0; i < aa.length; ++i) {
     System.out.println(aa[i]);
}

3 for each方法

for (int i: aa) {
     System.out.println(i);
}

三. 使用itertator的方法(用到InnerClass)

InnerClass在这里可以理解为是OuterClass的helper function

public class DataStructure {
    
    // Create an array
    private final static int SIZE = 15;
    private int[] arrayOfInts = new int[SIZE];
    
    public DataStructure() {
        // fill the array with ascending integer values
        for (int i = 0; i < SIZE; i++) {
            arrayOfInts[i] = i;
        }
    }
    
    public void printEven() {
        
        // Print out values of even indices of the array
        DataStructureIterator iterator = this.new EvenIterator();
        while (iterator.hasNext()) {
            System.out.print(iterator.next() + " ");
        }
        System.out.println();
    }
    
    interface DataStructureIterator extends java.util.Iterator<Integer> { } 


    // Inner class implements the DataStructureIterator interface,
    // which extends the Iterator<Integer> interface
    
    private class EvenIterator implements DataStructureIterator {
        
        // Start stepping through the array from the beginning
        private int nextIndex = 0;
        
        public boolean hasNext() {
            
            // Check if the current element is the last in the array
            return (nextIndex <= SIZE - 1);
        }        
        
        public Integer next() {
            
            // Record a value of an even index of the array
            Integer retValue = Integer.valueOf(arrayOfInts[nextIndex]);
            
            // Get the next even element
            nextIndex += 2;
            return retValue;
        }
    }
    
    public static void main(String s[]) {
        
        // Fill the array with integer values and print out only
        // values of even indices
        DataStructure ds = new DataStructure();
        ds.printEven();
    }
}

The output is:
0 2 4 6 8 10 12 14 


四. 打印二维数组

public class FFChap7 {
    public static void main(String[] args) {
    // TODO Auto-generated method stub
    int [][] numArray = new int [5][5];
        for (int i = 0; i < 5;++i) {
            for (int j = 0; j < 5;++j) {
                numArray[i][j]++;
            }
        }
    System.out.println(Arrays.deepToString(numArray));
    }
}

五. 打印Arraylist

ArrayList alist = new ArrayList(5);
alist.add(10);
alist.add(11);
alist.add(12);
alist.add(13);
System.out.println(alist);

参考资料：

https://docs.oracle.cm/javase/tutorial/java/javaOO/innerclasses.html