对象列表中元素的相等性检查

Question

I have a List of Employee object.

class Employee{  
  private int empId;  
  private String name;  
}  

Now I have

List<Employee> empList = new ArrayList<Employee>();  

How can I find, if my list contains an employee named "ABC"??
empList.contains("ABC"); wont work...

Should I put it in Map ?? Which one is more efficient??

Just wanted to mention that I get my Employee object from database....

Answer 1

You can use

Map<String, Employee> map = new HashMap<>();
map.put("ABC", new Employee("ABC"));
map.put("John", new Employee("John"));

and then check

map.containsKey("ABC")

Should I put it in Map?? Which one is more efficient??

Because contains() method of list, calls indexOf , which needs to iterate over all elements like this

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

Where as map no need to iterate over all elements

Answer 2

Override equals. You can then use List.contains

class Employee {  
    private empId;  
    private name;
    public boolean equals(Object o) {
        return (o instanceof Employee && ((Employee)o).empId == empId && ((Employee)o).name = name);
    }
}  

List l = ...;
Employee e = new Employee(...);
l.add(e);
l.contains(e);

Answer 3

Since you are storing the Employee objects and not String in your list , i think it is impossible to search without looping through all list objects

for (Employee employee : empList) {
      if (employee.getName().equals(searchString))
        System.out.println("Found");
    }

Note: Your Employee class should give access to name field either through getter method or change it to public

---

There are other alternatives, but it depends on your requirements and tradeoff's between speed, space, readability, resources etc

One thing i can think of is HashMap , which has constant time lookup in average case

HashMap<Integer, String> hm = new HashMap<Integer, String>();
hm.put(1, "Tom");
System.out.println(hm.containsValue("Tom"));

Now,

Should I put it in Map?? Which one is more efficient??

Instead of coding and analyzing, Know Thy Complexities beforehand !

Answer 4

在Java 8中，如果要确定员工列表中是否包含名为“ ABC”的员工，则可以执行以下操作：

boolean containsABC = empList.stream().anyMatch(emp -> emp.getName().equals("ABC"));

Answer 5

Here is the code that you can use. I am considering that you want list to return true when empId and name of the Employee matches.
I also prefer to use Constructor in your code(Just recommendation).
The below code will run as you are wanting it to be.

class Employee {

    private int empId;
    private String name;

    // below overriden function will return true if it found Employee with 
    // same ID and name
    @Override
    public boolean equals(Object obj) {
        return (obj instanceof Employee             //Checking instace of obj
            && ((Employee)obj).empId == empId       //Checking empId
            && ((Employee)obj).name.equals(name));  //Checking name
    }

    // Used constructor to create Employee
    Employee(int id, String nm) {
        empId = id;
        name = nm;
    }

}

Here is an example run :

List l = new ArrayList();
l.add(new Employee(1, "ME");
System.out.println(l.contains(new Employee(1, "ME")));  //print true

I would also like to acknowledge you that you should also override hashCode() when you decides to override equals(...) method according to Design Pattern .