Let’s see how to get a JSON string based on the attributes of an Apex class in Salesforce

public class MyClass {
    public String field1;
    public String field2;
    
    public String getJson(){
        JSONGenerator gen = JSON.createGenerator(false);
        gen.writeStartObject();
        if(this.field1 != null) gen.writeStringField('field1', this.field1);
        if(this.field2 != null) gen.writeStringField('field2', this.field2);
        gen.writeEndObject();
        return gen.getAsString();
    }
}