4.3 Array Creation and Access

1. Exam Points

• Create an array (length can be 0, but must be specified).
• When an array is created, all of its elements are initialized to the default values for the type of the array.
• Access an element of an 1D array using an array name and an index. nums[0]
• Traverse an array of primitive values.
• Traverse an array of object references.
• Valid index of an 1D array: 0 to length-1.
• Pass an array as an argument into a method.
• A method can return an array as a result.
• The length attribute of an array is final, it can not be changed.
• Note: how the index changes (i++, i+=2, i--)

2. Knowledge Points

(1) Understand and Create Arrays

• An array stores multiple values of the same type.
• The values can be either primitive values or object references.
• Examples:
  

  // Arrays of primitive values
  int[] numbers = new int[5];
  boolean[] flags = new boolean[6];
  double[] prices = new double[10];
        
  // Arrays of object references
  Dog[] dogs = new Dog[10];
  Student[] students = new Student[20];
  

• The length of an array is established at the time of creation and cannot be changed. (fixed length)
• The length of an array can be accessed through the length attribute.
  • Example: numbers.length
• When an array is created using the keyword new, all of its elements are initialized to the default values for the element data type.
  • The default value is:
    • int : 0
    • double : 0.0
    • boolean : false
    • reference type (ex. String, Student, Dog) : null
• Two ways to create an array:
  • using the new keyword: Ex. int[] numbers= new int[5];
  • using an initializer list: Ex. int[] nums = { 11, 22, 33, 44, 55 };

(2) Access Array Elements

• Access an element in one 1D array: arrayName[index] .
  • Example: numbers[0]
• The valid index values for an array are 0 to length-1.
• Using an index value outside of this range will result in an ArrayIndexOutOfBoundsException.

(3) Arrays as Parameters or Return Types

• An array can be passed into a method as an argument(实际参数). (arrays as parameters)
  • Example:
    

  // Here, the method takes a double array as a parameter(形式参数)
  static double sum(double[] array) {
      double sum = 0.0;
      for (int i = 0; i < array.length; i++) {
          sum += array[i];
      }
      return sum;
  }
      
  public static void main(String[] args) {
      double[] prices = {1.2, 3.5, 6.7, 9.0};
      // here, the array 'prices' is passed into the 'sum' method as an argument(实际参数).
      double result = sum(prices);
  }
  

• An array can be returned as the result in a method. (arrays as return types)
  • Example:
    

  // here, this method takes a double array as a parameter, 
  // and returns another array as the result.
  static double[] changePrices(double[] array) {
      double[] newArray = new double[array.length];
      for (int i = 0; i < array.length; i++) {
          newArray[i] = array[i] * 0.8;
      }
      return newArray;
  }
  public static void main(String[] args) {
      double[] prices = {1.2, 3.5, 6.7, 9.0};
      // here we use a variable of the 'double[]' type (same as the return type) 
      // to accept the returned result of the 'changePrices' method.
      double[] anotherArray = sum(prices);
  }
  

(4) Traverse 1D Arrays

• Traverse an 1D array of primitive values.
  • Example:
    

    int[] nums = { 1, 2, 3, 4 };
    // 1. for loop (use indexes)
    for (int i = 0; i < nums.length; i++) { 
        // i is an index, nums[i] is an element
        System.out.print(nums[i] + " ");
    }
    
    System.out.println();
    
    // 2. for-each loop (no use of indexes)
    for (int x : nums) { // here type 'int' follows the type of the array
        // here x is the current element
        System.out.print(x + " ");
    }
    

• Traverse an 1D array of object references.
  • Example:
    

    class Pig {
        
        private String name;
        private boolean vaccinated;
        
        public Pig(String n, boolean v) {
            name = n;
            vaccinated = v;
        }
        
        public String getName() {
            return name;
        }
        
        public boolean isVaccinated() {
            return vaccinated;
        }
    }
    

    // 1. use for loops (use indexes)
    for (int i = 0; i < pigs.length; i++) {
        System.out.println(pigs[i].getName() + "-" + pigs[i].isVaccinated());
    }
    
    // 2. use for-each loops (no use of indexes)
    for (Pig p : pigs) {
        System.out.println(p.getName() + "-" + p.isVaccinated());
    }
    

3. Exercises

• Practice
• Solution