大多数时我们要将自定义的对象存入到集合中，在操作自定义对象时常会遇到的问题。
1. 首先是使用普通for循环遍历对象时，将满足条件的对象删除等操作。
if(26 == list.get(i).getAge())
list.remove(i);
删除后发现结果用仍有年龄为26的对象被保留下来，这是为什么呢？参见下图。是因为在遍历时有的对象没有被判断到。
package com.test.list;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class AddCustomElements
{
public static void main(String[] args)
{
List<Student> list = new ArrayList<Student>();
list.add(new Student("zhangsan", 26));
list.add(new Student("lisi", 26));
list.add(new Student("wangwu",30));
list.add(new Student("niuqi", 26));
//如果要将年龄为26的元素删除使用普通for循环和Iterator迭代器有点区别：
for(int i = 0; i < list.size(); i++)
{
if(26 == list.get(i).getAge())
//list.remove(i);
//为了避免有漏掉的对象
list.remove(i--);
}
System.out.println("For: "+list);
//使用迭代器就可以将所有满足条件的对象删除
for(Iterator<Student> it = list.iterator(); it.hasNext();)
{
if(26 == it.next().getAge())
it.remove();
}
System.out.println("Iterator: "+list);
}
}
class Student
{
private String name;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
private int age;
public Student()
{
}
public Student(String name, int age)
{
this.name = name;
this.age = age;
}
public String toString()
{
return name.toString()+", "+ age;
}
}
2. 如果是同年龄，同名字的Student就看成是同一个对象，要在集合中删除相同元素时，就要复写Student类中的equals()方法。
package com.test.list;
import java.util.ArrayList;
import java.util.List;
public class CustomElements
{
public static void main(String[] args)
{
List<Student> list = new ArrayList<Student>();
list.add(new Student("zhangsan", 24));
list.add(new Student("lisi", 29));
list.add(new Student("zhangsan", 24));
list.add(new Student("zhaoliu", 30));
List<Student> tempList = singleElements(list);
for(Student stu: tempList)
{
System.out.println(stu.getName()+", "+stu.getAge());
}
}
//定义一个方法，将List集合中的重复元素去掉
public static <T> List<T> singleElements(List<T> list)
{
List<T> tlist = new ArrayList<T>();
if (list == null || list.isEmpty())
{
return tlist;
}
for (T t : list)
{
if (!tlist.contains(t))
{
tlist.add(t);
}
}
return tlist;
}
}
class Student
{
private String name;
private int age;
public Student()
{
}
public Student(String name, int age)
{
this.name = name;
this.age = age;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
//如果不复写equals()方法，就不知道什么样的元素是相同的
public boolean equals(Object obj)
{
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Student other = (Student) obj;
if (age != other.age)
return false;
if (name == null)
{
if (other.name != null)
return false;
}
else if (!name.equals(other.name))
return false;
return true;
}
}
因为复写了equals()方法，可以使用indexOf(), lastIndexOf()方法等
int index = list.lastIndexOf(new Student("zhangsan", 24));
System.out.println("index = "+ index);