Java AbstractList indexOf()方法及示例

java.util.AbstractList 类的 indexOf() 方法用于返回指定元素在这个列表中第一次出现的索引，如果这个列表不包含该元素，则返回-1。更正式的说法是，返回最低的索引i，使得(o==null ? get(i)==null : o.equals(get(i)))，如果没有这样的索引，则返回-1。

语法

public int indexOf(Object o)

参数： 该方法以Object o为参数，它是要搜索的元素。

返回值： 该方法返回指定元素在这个列表中第一次出现的索引，如果这个列表不包含该元素，则返回-1。

下面是说明 indexOf() 方法的例子。

例1 :

// Java program to demonstrate
// indexOf() method
// for Integer value
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {
  
        try {
  
            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();
  
            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);
  
            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);
  
            // getting the index of element 30
            // using indexOf() method
            int index = arrlist1.indexOf(30);
  
            // print the index
            System.out.println("index : " + index);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

ArrayListlist : [10, 20, 30, 40, 50]
index : 2

例2 :

// Java program to demonstrate
// indexOf() method
// for Integer value
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {
  
        try {
  
            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();
  
            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);
  
            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);
  
            // getting the index of element 25
            // using indexOf() method
            int index = arrlist1.indexOf(25);
  
            // print the index
            System.out.println("Index of 25 : "
                               + index);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

ArrayListlist : [10, 20, 30, 40, 50]
Index of 25 : -1

例3： 对于空值

// Java program to demonstrate
// indexOf() method
// for null value
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {
  
            // Creating object of AbstractList<Integer>
            AbstractList<Integer>
                arrlist1 = new ArrayList<Integer>();
  
            // Populating arrlist1
            arrlist1.add(10);
            arrlist1.add(20);
            arrlist1.add(30);
            arrlist1.add(40);
            arrlist1.add(50);
  
            // print arrlist1
            System.out.println("ArrayListlist : "
                               + arrlist1);
  
            // creating null object
            Object value = null;
  
            // getting the index of element null
            // using indexOf() method
            int index = arrlist1.indexOf(value);
  
            // print the index
            System.out.println("Index of null : "
                               + index);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

ArrayListlist : [10, 20, 30, 40, 50]
Index of null : -1