Composition of class in Java

// "static void main" must be defined in a public class.
class OS {
    String name;
    int version;
    OS(String name, int version) {
        this.name = name;
        this.version = version;
    }
    
    @Override
    public String toString() {
        return "\nOS details\n name= "+name+", version= "+version;
    }
}

class CPU {
    String name;
    int RAM;
    int ROM;
    CPU(String name, int RAM, int ROM) {
        this.name = name;
        this.RAM = RAM;
        this.ROM = ROM;
    }
    
    @Override
    public String toString() {
        return "\nCPU details\n name= "+name+", RAM= "+RAM+", ROM= "+ROM;
    }
}

public class Computer {
    String name;
    int price;
    OS os;
    CPU cpu;
    
    Computer(String name, int price, OS os) {
        this.name = name;
        this.price = price;
        this.os = os;
    }
    
    @Override
    public String toString() {
        return "\nComputer Details\n name= "+name+", price= "+price+", OS= "+os+", CPU= "+cpu;
    }
    
    public static void main(String[] args) {
        OS osObj = new OS("Windows", 10);
        CPU cpuObj = new CPU("Inter Core i-7", 16, 512);
        System.out.println(cpuObj);
        Computer computer = new Computer("Dell", 32000, osObj);
        System.out.println(computer);
    }
}