I created a function that gets an array of objects and the type(String) of the abject and returning an array of objects with a specific type and after that, I tried to print it but it only print the first 1, not sure what went wrong. The objects are Tables:

public class q2Table {
private String color;
private int length;
private int width;
private int height;
private String type;

//I made a full contractor:

public q2Table(String color, int length, int width, int height, String type) {
    
    this.color = color;
    this.length = length;
    this.width = width;
    this.height = height;
    this.type = type;
}

//and a setter getter for each :

public String getColor() {
    return color;
}
public void setColor(String color) {
    this.color = color;
}
public int getLength() {
    return length;
}
public void setLength(int length) {
    this.length = length;
}
public int getWidth() {
    return width;
}
public void setWidth(int width) {
    this.width = width;
}
public int getHeight() {
    return height;
}
public void setHeight(int height) {
    this.height = height;
}
public String getType() {
    return type;
}
public void setType(String type) {
    this.type = type;
}

//I created a main class and also and random array of objects:

q2Table[] myTable = new q2Table[5];
        myTable[0] = new q2Table("Yellow" , 20 , 20 , 20 , "Round");
        myTable[1] = new q2Table("White" , 22 , 22 , 22 , "Rectangle");
        myTable[2] = new q2Table("Purple" , 21 , 21 , 21 , "Round");
        myTable[3] = new q2Table("Blue" , 21 , 21 , 21 , "Traingle");
        myTable[4] = new q2Table("Green" , 21 , 21 , 21 , "Round");

Created the function I asked about in the beginning :

public static q2Table[] tableTypeArr(q2Table[] tables,String t) {
        // Getting the index i need for the returning array
        String type = t;
        int i  , x=0  , j;
        for(i=0;i<tables.length;i  ) {
            if(tables[i].getType()==type) {
                x  ;
            }
        }
        //creating the returning array
        q2Table[] types = new q2Table[x];
        
        for(j=0;j<types.length;j  ) {
            if(tables[j].getType()==type) {
                types[j]=tables[j];
            }
        }
      return types;
    }

//created two Arrays of objects, 1 with random objects that suppose to enter the function and a //receiving //Array of objects to print:

q2Table[] testTable = new q2Table[3];
        testTable = tableTypeArr(myTable , "Round");
        int i;
        for(i=0;i<testTable.length;i  ) {
            System.out.println(testTable[i].getColor()   " "  testTable[i].getHeight()   " " testTable[i].getLength() " "  testTable[i].getWidth() " " testTable[i].getType() );
        }

//I successfully printed the first object and after that got an error in the terminal:

Yellow 20 20 20 Round Exception in thread "main" java.lang.NullPointerException: Cannot invoke "lab4.q2Table.getColor()" because "testTable[i]" is null at lab4.q2Main.main(q2Main.java:70)

CodePudding user response：

The NullPointerException occurs because in the second for loop in the function, you're just going through the first x number of tables. You need to loop through all tables and select the matching typed tables into the new array.

public static q2Table[] tableTypeArr(q2Table[] tables,String t) {
    // Getting the index i need for the returning array
    String type = t;
    int i  , x=0  , j;
    for(i=0;i<tables.length;i  ) {
        if(tables[i].getType()==type) {
            x  ;
        }
    }
    //creating the returning array
    q2Table[] types = new q2Table[x];
    i=0; // Use i to maintain index for the new array
    for(j=0;j<tables.length;j  ) { // Loop all tables
        if(tables[j].getType()==type) {
            types[i  ]=tables[j]; // Set the ith index value to the matched table and increment i.
        }
    }
    return types;
}

CodePudding user response：

Your String comparison is wrong: you do: tables[j].getType()==type you should do: tables[j].getType().equals(type). So when you fill your array in your tableTypeArr you only pick one record and not 3 as you thought. So, when you try to iterate through your testTable, it only holds one record and 2 null values. So you print the first and on the second element you get your NPE (NUllPointerException). You can verify all this in debug.