The Singleton pattern attempts to solve the issue of repeatedly using an object instance, but only allowing to instantiate it once within a single transaction context. So, using Singleton pattern , we can create Salesforce Apex Class to create one object per transaction.

Sample Class:

public class SampleClass {

    private static SampleClass objInstance  = null;
    public String str1;
    public String str2;

    private SampleClass() {
        
        str1 = 'String1';
        str2 = 'String2';
        System.debug( 'Inside Constructor' );
        
    }

    public static SampleClass getInstance () {
    
        if (objInstance  == null) {
            
            objInstance  = new SampleClass();
            
        }
        
        return objInstance ;
        
    }

}

To test, execute the following the code:

for ( Integer i = 0; i < 3; i++ ) {
    
    SampleClass obj = SampleClass.getInstance();
    System.debug( 'str1 is ' + obj.str1 );
    System.debug( 'str2 is ' + obj.str2 );
    
}

Output:

In the above example, Constructor was called only once. So, the object was initialised only once.