Check object in list Java

• Reference

Namespace: System.Collections.Generic Assembly:System.Collections.dll Assembly:mscorlib.dll Assembly:netstandard.dll

Important

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public: virtual bool Contains[T item]; public bool Contains [T item]; abstract member Contains : 'T -> bool override this.Contains : 'T -> bool Public Function Contains [item As T] As Boolean

Parameters

Returns

Boolean

true if item is found in the List; otherwise, false.

Implements

Examples

The following example demonstrates the Contains and Exists methods on a List that contains a simple business object that implements Equals.

using System; using System.Collections.Generic; // Simple business object. A PartId is used to identify a part // but the part name can change. public class Part : IEquatable { public string PartName { get; set; } public int PartId { get; set; } public override string ToString[] { return "ID: " + PartId + " Name: " + PartName; } public override bool Equals[object obj] { if [obj == null] return false; Part objAsPart = obj as Part; if [objAsPart == null] return false; else return Equals[objAsPart]; } public override int GetHashCode[] { return PartId; } public bool Equals[Part other] { if [other == null] return false; return [this.PartId.Equals[other.PartId]]; } // Should also override == and != operators. } public class Example { public static void Main[] { // Create a list of parts. List parts = new List[]; // Add parts to the list. parts.Add[new Part[] { PartName = "crank arm", PartId = 1234 }]; parts.Add[new Part[] { PartName = "chain ring", PartId = 1334 }]; parts.Add[new Part[] { PartName = "regular seat", PartId = 1434 }]; parts.Add[new Part[] { PartName = "banana seat", PartId = 1444 }]; parts.Add[new Part[] { PartName = "cassette", PartId = 1534 }]; parts.Add[new Part[] { PartName = "shift lever", PartId = 1634 }]; ; // Write out the parts in the list. This will call the overridden ToString method // in the Part class. Console.WriteLine[]; foreach [Part aPart in parts] { Console.WriteLine[aPart]; } // Check the list for part #1734. This calls the IEquatable.Equals method // of the Part class, which checks the PartId for equality. Console.WriteLine["\nContains: Part with Id=1734: {0}", parts.Contains[new Part { PartId = 1734, PartName = "" }]]; // Find items where name contains "seat". Console.WriteLine["\nFind: Part where name contains \"seat\": {0}", parts.Find[x => x.PartName.Contains["seat"]]]; // Check if an item with Id 1444 exists. Console.WriteLine["\nExists: Part with Id=1444: {0}", parts.Exists[x => x.PartId == 1444]]; /*This code example produces the following output: ID: 1234 Name: crank arm ID: 1334 Name: chain ring ID: 1434 Name: regular seat ID: 1444 Name: banana seat ID: 1534 Name: cassette ID: 1634 Name: shift lever Contains: Part with Id=1734: False Find: Part where name contains "seat": ID: 1434 Name: regular seat Exists: Part with Id=1444: True */ } } Imports System.Collections.Generic ' Simple business object. A PartId is used to identify a part ' but the part name can change. Public Class Part Implements IEquatable[Of Part] Public Property PartName[] As String Get Return m_PartName End Get Set[value As String] m_PartName = Value End Set End Property Private m_PartName As String Public Property PartId[] As Integer Get Return m_PartId End Get Set[value As Integer] m_PartId = Value End Set End Property Private m_PartId As Integer Public Overrides Function ToString[] As String Return Convert.ToString["ID: " & PartId & " Name: "] & PartName End Function Public Overrides Function Equals[obj As Object] As Boolean If obj Is Nothing Then Return False End If Dim objAsPart As Part = TryCast[obj, Part] If objAsPart Is Nothing Then Return False Else Return Equals[objAsPart] End If End Function Public Overrides Function GetHashCode[] As Integer Return PartId End Function Public Overloads Function Equals[other As Part] As Boolean _ Implements IEquatable[Of Part].Equals If other Is Nothing Then Return False End If Return [Me.PartId.Equals[other.PartId]] End Function ' Should also override == and != operators. End Class Public Class Example Public Shared Sub Main[] ' Create a list of parts. Dim parts As New List[Of Part][] ' Add parts to the list. parts.Add[New Part[] With { _ .PartName = "crank arm", _ .PartId = 1234 _ }] parts.Add[New Part[] With { _ .PartName = "chain ring", _ .PartId = 1334 _ }] parts.Add[New Part[] With { _ .PartName = "regular seat", _ .PartId = 1434 _ }] parts.Add[New Part[] With { _ .PartName = "banana seat", _ .PartId = 1444 _ }] parts.Add[New Part[] With { _ .PartName = "cassette", _ .PartId = 1534 _ }] parts.Add[New Part[] With { _ .PartName = "shift lever", _ .PartId = 1634 _ }] ' Write out the parts in the list. This will call the overridden ToString method ' in the Part class. Console.WriteLine[] For Each aPart As Part In parts Console.WriteLine[aPart] Next ' Check the list for part #1734. This calls the IEquatable.Equals method ' of the Part class, which checks the PartId for equality. Console.WriteLine[vbLf & "Contains: Part with Id=1734: {0}", parts.Contains[New Part[] With { _ .PartId = 1734, _ .PartName = "" _ }]] ' Find items where name contains "seat". Console.WriteLine[vbLf & "Find: Part where name contains ""seat"": {0}", parts.Find[Function[x] x.PartName.Contains["seat"]]] ' Check if an item with Id 1444 exists. Console.WriteLine[vbLf & "Exists: Part with Id=1444: {0}", parts.Exists[Function[x] x.PartId = 1444]] 'This code example produces the following output: ' ' ID: 1234 Name: crank arm ' ID: 1334 Name: chain ring ' ID: 1434 Name: regular seat ' ID: 1444 Name: banana seat ' ID: 1534 Name: cassette ' ID: 1634 Name: shift lever ' ' Contains: Part with Id=1734: False ' ' Find: Part where name contains "seat": ID: 1434 Name: regular seat ' ' Exists: Part with Id=1444: True ' End Sub End Class

The following example contains a list of complex objects of type Cube. The Cube class implements the IEquatable.Equals method so that two cubes are considered equal if their dimensions are the same. In this example, the Contains method returns true, because a cube that has the specified dimensions is already in the collection.

using System; using System.Collections.Generic; class Program { static void Main[string[] args] { List cubes = new List[]; cubes.Add[new Cube[8, 8, 4]]; cubes.Add[new Cube[8, 4, 8]]; cubes.Add[new Cube[8, 6, 4]]; if [cubes.Contains[new Cube[8, 6, 4]]] { Console.WriteLine["An equal cube is already in the collection."]; } else { Console.WriteLine["Cube can be added."]; } //Outputs "An equal cube is already in the collection." } } public class Cube : IEquatable { public Cube[int h, int l, int w] { this.Height = h; this.Length = l; this.Width = w; } public int Height { get; set; } public int Length { get; set; } public int Width { get; set; } public bool Equals[Cube other] { if [this.Height == other.Height && this.Length == other.Length && this.Width == other.Width] { return true; } else { return false; } } } Imports System.Collections.Generic Class Program Public Shared Sub Main[ByVal args As String[]] Dim cubes As New List[Of Cube][] cubes.Add[New Cube[8, 8, 4]] cubes.Add[New Cube[8, 4, 8]] cubes.Add[New Cube[8, 6, 4]] If cubes.Contains[New Cube[8, 6, 4]] Then Console.WriteLine["An equal cube is already in the collection."] Else Console.WriteLine["Cube can be added."] End If 'Outputs "An equal cube is already in the collection." End Sub End Class Public Class Cube Implements IEquatable[Of Cube] Public Sub New[ByVal h As Integer, ByVal l As Integer, ByVal w As Integer] Me.Height = h Me.Length = l Me.Width = w End Sub Private _Height As Integer Public Property Height[] As Integer Get Return _Height End Get Set[ByVal value As Integer] _Height = value End Set End Property Private _Length As Integer Public Property Length[] As Integer Get Return _Length End Get Set[ByVal value As Integer] _Length = value End Set End Property Private _Width As Integer Public Property Width[] As Integer Get Return _Width End Get Set[ByVal value As Integer] _Width = value End Set End Property Public Overloads Function Equals[ByVal other As Cube] _ As Boolean Implements IEquatable[Of Cube].Equals If Me.Height = other.Height And Me.Length = other.Length _ And Me.Width = other.Width Then Return True Else Return False End If End Function End Class

Remarks

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable.Equals method for T [the type of values in the list].

This method performs a linear search; therefore, this method is an O[n] operation, where n is Count.

Applies to

See also

• IndexOf[T]
• LastIndexOf[T]
• Performing Culture-Insensitive String Operations in Collections

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