检查 ArrayList 中是否存在值

Question

How can I check if a value exists in an ArrayList ?

List<CurrentAccount> lista = new ArrayList<CurrentAccount>();

CurrentAccount conta1 = new CurrentAccount("Alberto Carlos", 1052);
CurrentAccount conta2 = new CurrentAccount("Pedro Fonseca", 30);
CurrentAccount conta3 = new CurrentAccount("Ricardo Vitor", 1534);
CurrentAccount conta4 = new CurrentAccount("João Lopes", 3135);

lista.add(conta1);
lista.add(conta2);
lista.add(conta3);
lista.add(conta4);

Collections.sort(lista);

System.out.printf("Bank Accounts:" + "%n");
Iterator<CurrentAccount> itr = lista.iterator();
while (itr.hasNext()) {
    CurrentAccount element = itr.next();
    System.out.printf(element + " " + "%n");
}
System.out.println();

Answer 1

Just use ArrayList.contains(desiredElement) . For example, if you're looking for the conta1 account from your example, you could use something like:

if (lista.contains(conta1)) {
    System.out.println("Account found");
} else {
    System.out.println("Account not found");
}

Edit: Note that in order for this to work, you will need to properly override the equals() and hashCode() methods. If you are using Eclipse IDE, then you can have these methods generated by first opening the source file for your CurrentAccount object and the selecting Source > Generate hashCode() and equals()...

Answer 2

Better to use a HashSet than an ArrayList when you are checking for existence of a value.

Java docs for HashSet says

This class offers constant time performance for the basic operations (add, remove, contains and size)

ArrayList.contains() might have to iterate the whole list to find the instance you are looking for.

Answer 3

We can use contains method to check if an item exists if we have provided the implementation of equals and hashCode else object reference will be used for equality comparison. Also in case of a list contains is O(n) operation where as it is O(1) for HashSet so better to use later. In Java 8 we can use streams also to check item based on its equality or based on a specific property.

Java 8

CurrentAccount conta5 = new CurrentAccount("João Lopes", 3135);
boolean itemExists = lista.stream().anyMatch(c -> c.equals(conta5)); //provided equals and hashcode overridden
System.out.println(itemExists); // true

String nameToMatch = "Ricardo Vitor";
boolean itemExistsBasedOnProp = lista.stream().map(CurrentAccount::getName).anyMatch(nameToMatch::equals);
System.out.println(itemExistsBasedOnProp); //true

Answer 4

Please refer to my answer on this post .

There is no need to iterate over the List ; just override the equals method and use .contains .

@Override
public boolean equals (Object object) {
    boolean result = false;
    if (object == null || object.getClass() != getClass()) {
        result = false;
    } else {
        EmployeeModel employee = (EmployeeModel) object;
        if (this.name.equals(employee.getName()) && this.designation.equals(employee.getDesignation())   && this.age == employee.getAge()) {
            result = true;
        }
    }
    return result;
}

Call it like this:

public static void main(String args[]) {

    EmployeeModel first = new EmployeeModel("Sameer", "Developer", 25);
    EmployeeModel second = new EmployeeModel("Jon", "Manager", 30);
    EmployeeModel third = new EmployeeModel("Priyanka", "Tester", 24);

    List<EmployeeModel> employeeList = new ArrayList<EmployeeModel>();
    employeeList.add(first);
    employeeList.add(second);
    employeeList.add(third);

    EmployeeModel checkUserOne = new EmployeeModel("Sameer", "Developer", 25);
    System.out.println("Check checkUserOne is in list or not");
    System.out.println("Is checkUserOne Preasent = ? " + employeeList.contains(checkUserOne));

    EmployeeModel checkUserTwo = new EmployeeModel("Tim", "Tester", 24);
    System.out.println("Check checkUserTwo is in list or not");
    System.out.println("Is checkUserTwo Preasent = ? " + employeeList.contains(checkUserTwo));

}

Answer 5

Just use .contains . For example, if you were checking if an ArrayList arr contains a value val , you would simply run arr.contains(val) , which would return a boolean representing if the value is contained. For more information, see the docs for .contains .

Answer 6

When Array List contains object of Primitive DataType.

Use this function:
arrayList.contains(value);

if list contains that value then it will return true else false.

When Array List contains object of UserDefined DataType.

Follow this below Link 

How to compare Objects attributes in an ArrayList?

I hope this solution will help you. Thanks

Answer 7

public static void linktest()
{
    System.setProperty("webdriver.chrome.driver","C://Users//WDSI//Downloads/chromedriver.exe");
    driver=new ChromeDriver();
    driver.manage().window().maximize();
    driver.get("http://toolsqa.wpengine.com/");
    //List<WebElement> allLinkElements=(List<WebElement>) driver.findElement(By.xpath("//a"));
    //int linkcount=allLinkElements.size();
    //System.out.println(linkcount);
    List<WebElement> link = driver.findElements(By.tagName("a"));
    String data="HOME";
    int linkcount=link.size();
    System.out.println(linkcount);
    for(int i=0;i<link.size();i++) { 
        if(link.get(i).getText().contains(data)) {
            System.out.println("true");         
        }
    } 
}

Answer 8

Below code can helps

List<CurrentAccount> lista = new ArrayList<CurrentAccount>();
CurrentAccount conta5 = new CurrentAccount("Pedro Fonseca", 500);
boolean isAdded = lista.contains(model);

ArrayList already have same name so it returns true

import com.google.gson.annotations.SerializedName;    
import java.util.Objects;

public class CurrentAccount {

    @SerializedName("Name")
    private String name;
    
    @SerializedName("Balance")
    private int balance;


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getBalance() {
        return balance;
    }

    public void setBalance(int balance) {
        this.balance = balance;
    }

    public CurrentAccount(String name, int balance) {
        this.name = name;
        this.balance = balance;
    }
    
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CurrentAccount model = (CurrentAccount) o;
        return name.equals(model.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }
}

Answer 9

使用 .contains 将值与