如何解决不兼容的类型：使用 arrays 在 java 中返回错误？

Question

Here is my unfinished code. I have arrays in the phone book class and the main is calling a method to print out everything in the arrays, but there is an error.

Main class:

import java.util.Scanner;
public class Main {
  public static void main(String[] args) {
    
    Scanner myObj = new Scanner(System.in);
    System.out.println("What would you like to do: \n 1) ADD to phone book \n 2) DELETE from phone book \n 3) CALL \n 4) PRINT phone book \n 5) quit");
    int input = myObj.nextInt();


    //constructor scanner inputs
    String ISpeedDial = myObj.nextLine();
    String Iname = myObj.nextLine();
    String Inumber = myObj.nextLine();
    //constructor
    PhoneBook b1 = new PhoneBook(ISpeedDial, Iname, Inumber);

    if (input == 1) {//add contact
// scanner saying what is the name of your new contact? what is the # of you new contact? Would you like this contact to be on your spead dails. 
//ptints your new contat is:____ and their # is ___-___-____ and the your Spead dial number is_

    }
    else if (input == 2) {//delete contact
// 
    }
    else if (input == 3) {//make call(from contact or not?)

    }
    else if (input == 4) {//print phone book
    b1.print();
    System.out.println(b1.print() + " ");
    //System.out.print(SD0[i] + " ");
    //t1.Right();//gets right triangle t/f
    //System.out.println("It is a right triangle: " + t1.Right());

    }
    else if (input == 5) {//quit
      return;
    }
    else {//invalid number
      System.out.println("invalid input: " + input);
      return;
    }

  }
}

PhoneBook class:

public class PhoneBook {

  //instance variables
  private String SpeedDial;
  private String name;
  private String number;

//Account constructor
public PhoneBook(String MSpeedDial, String Mname, String Mnumber){

SpeedDial = MSpeedDial;
name = Mname;
number = Mnumber;
}

//arrays
  String [] SD0 = new String[3];
  String [] SD1 = new String[3];
  String [] SD2 = new String[3];
  String [] SD3 = new String[3];
  String [] SD4 = new String[3];
  String [] SD5 = new String[3];
  String [] SD6 = new String[3];
  String [] SD7 = new String[3];
  String [] SD8 = new String[3];
  String [] SD9 = new String[3];

//method to deposit a specified amount into the account
public void add(){




  SD0[0] = "0";
  SD0[1] = "jo";
  SD0[2] = "123";

}

public void delete(){
  
}
//getter method to return balance
//named without void because it will return a value unlike the ones above
public String print(){
  for (int i = 0; i < SD0.length; i++) {  
    return SD0[i];  
  }
  for (int i = 0; i < SD1.length; i++) {  
    return SD1[i];  
  }
  for (int i = 0; i < SD2.length; i++) {  
    return SD2[i];  
  }
  for (int i = 0; i < SD3.length; i++) {  
    return SD3[i];
  }
  for (int i = 0; i < SD4.length; i++) {  
    return SD4[i];  
  }
  for (int i = 0; i < SD5.length; i++) {  
    return SD5[i];  
  }
  for (int i = 0; i < SD6.length; i++) {  
    return SD6[i];  
  }
  for (int i = 0; i < SD7.length; i++) {  
    return SD7[i]; 
  }
  for (int i = 0; i < SD8.length; i++) {  
    return SD8[i];  
  }
  for (int i = 0; i < SD9.length; i++) {  
    return SD9[i];  
  } 
  return;
}

}

My error is:

PhoneBook.java:76: error: incompatible types: missing return value
  return;

Does any one know how to fix that issue?

Also, is there a way to shorten lines(46-75) in the phone book class, of for-loops that prints each array?

Thank you!

Answer 1

Your last return in the print method returns nothing, when String is expected:

  ...
  for (int i = 0; i < SD9.length; i++) {  
    return SD9[i];  
  } 
  return;

You can consider using String [][] SD = new String[10][3]; instead of having all the SDx as a variable

Answer 2

The return statement returns immediately. It's not building a string. So your code is going to return SD0[0] as long as SD0 has length > 0.

The error is the final return with no argument at all. return "" will fix the compile error, but it probably isn't doing what you really want.