Thread safe Builder Pattern

I want to make my Builder pattern as Thread safe But facing issues in that, below is my code:

// Server Side Code 
final class Student { 
  
    // final instance fields 
    private final int id; 
    private final String name; 
    private final String address; 
  
    public Student(Builder builder) 
    { 
        this.id = builder.id; 
        this.name = builder.name; 
        this.address = builder.address; 
    } 
  
    // Static class Builder 
    public static class Builder {
  
        /// instance fields 
        private int id; 
        private String name; 
        private String address; 
  
        public static Builder newInstance() 
        { 
            return new Builder(); 
        } 
  
        private Builder() {} 
  
        // Setter methods 
        public Builder setId(int id) 
        { 
            this.id = id; 
            return this; 
        } 
        public Builder setName(String name) 
        { 
            this.name = name; 
            return this; 
        } 
        public Builder setAddress(String address) 
        { 
            this.address = address; 
            return this; 
        } 
  
        // build method to deal with outer class 
        // to return outer instance 
        public Student build() 
        { 
            return new Student(this); 
        } 
    } 
  
    @Override
    public String toString() 
    { 
        return "id = " + this.id + ", name = " + this.name +  
                               ", address = " + this.address; 
    } 
} 
  


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There is another class named StudentReceiver.java in which I am using multithreading:

    class StudentReceiver {

    // volatile student instance to ensure visibility
    // of shared reference to immutable objects
    private volatile Student student;

    public StudentReceiver() throws InterruptedException {

        Thread t1 = new Thread(new Runnable() {
            public void run() {
                student = Student.Builder.newInstance().setId(1).setName("Ram").setAddress("Noida").build();
            }
        });

        Thread t2 = new Thread(new Runnable() {
            public void run() {
                student = Student.Builder.newInstance().setId(2).setName("Shyam").setAddress("Delhi").build();
            }
        });

        t1.start();
        t2.start();
        //t1.join();
        //t2.join();
        
    }

    public Student getStudent() {
        return student;
    }
}


----------

Main class is below from where I am calling these methods:

//Driver class 
public class BuilderDemo { 
 public static void main(String args[]) throws InterruptedException 
 { 
     for(int i=0; i<10;i++)
     {
         StudentReceiver sr = new StudentReceiver();
         
         System.out.println(sr.getStudent());
     }
 } 
}


----------

The output I am getting is like below:

null
null
null
null
null
null
null
null
id = 1, name = Ram, address = Noida
null

Why I am getting null here?? May anyone explain and How to make Builder Pattern thread safe so that it can be used in multithreaaded environment.