Assignment Statements

An assignment statement is any statement that places a value into a variable, generally in one of the following two forms:

1[LValue] = [new_value_expression];
2[LValue] = [[inline_soql_query]];

In the forms above, [LValue] stands for any expression that can be placed on the left side of an assignment operator. These include:

A simple variable. For example:

1Integer i = 1;
2Account a = new Account();
3Account[] accts = [SELECT Id FROM Account];

A de-referenced list element. For example:
1ints[0] = 1; 2accts[0].Name = 'Acme';

An sObject field reference that the context user has permission to edit. For example:

1Account a = new Account(Name = 'Acme', BillingCity = 'San Francisco'); 
2
3// IDs cannot be set prior to an insert call
4// a.Id = '00300000003T2PGAA0';   
5
6// Instead, insert the record. The system automatically assigns it an ID.
7insert a;
8
9// Fields also must be writeable for the context user
10// a.CreatedDate = System.today();  This code is invalid because
11//                                  createdDate is read-only! 
12
13// Since the account a has been inserted, it is now possible to
14// create a new contact that is related to it
15Contact c = new Contact(LastName = 'Roth', Account = a);
16
17// Notice that you can write to the account name directly through the contact 
18c.Account.Name = 'salesforce.com';

Assignment is always done by reference. For example:

1Account a = new Account();
2Account b;
3Account[] c = new Account[]{};
4a.Name = 'Acme';
5b = a;         
6c.add(a);      
7
8// These asserts should now be true. You can reference the data
9// originally allocated to account a through account b and account list c.
10System.assertEquals(b.Name, 'Acme');          
11System.assertEquals(c[0].Name, 'Acme');

Similarly, two lists can point at the same value in memory. For example:

1Account[] a = new Account[]{new Account()};
2Account[] b = a;
3a[0].Name = 'Acme';
4System.assert(b[0].Name == 'Acme');

In addition to =, other valid assignment operators include +=, *=, /=, |=, &=, ++, and --. See Understanding Expression Operators.

Force.com Apex Code Developer's Guide

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Assignment Statements