Summary
Overview: 2.10.0
- Learned to use objects that have been written already
- Objects are variables of a class type
- Learned to use constructors and methods with and without parameters
- Learned to use built-in classes like
String
,Integer
,Double
andMath
Concept Summary: 2.10.1
class
- Defines a new data type
- A formal implementation of attributes and behaviors for objects of that class
object
- A specific instance of a class
- Declared as variables of a class type
constructor
- Code used to create a new object and initialize attributes
new
- keyword used to create an object and call one of the class’ constructors
instance variables
- Attributes for an object
methods
- Functions for objects
dot operator
- Used to access an object’s methods
parameters
- values passed to an object’s methods
- this is the name for the formal declaration
arguments
- The values passed as parameters when a method is actually called
return values
- Values returned by a method after being called
Immutable
- Not able to be changed
- Strings are immutable
- All methods which modify a string return a new string
- they do not modify the existing string itself
- All methods which modify a string return a new string
wrapper classes
- Classes which create objects from primitive types
Keyword Summary: 2.10.2
new
- used to create a new object
null
- used to indicate that an object reference doesn’t refer to anything
String methods and constructors
String(String str)
- Constructs a new String object that represents the same sequence of characters as str.
int length()
- returns the number of characters in a String object.
String substring(int from, int to)
- returns the substring beginning at index from and ending at index (to -1)
- The single element substring at position index can be created by calling substring(index, index + 1).
String substring(int from)
- returns substring(from, length()).
int indexOf(String str)
- returns the index of the first occurrence of str
- returns -1 if not found.
- returns the index of the first occurrence of str
boolean equals(String other)
- returns true if this (the calling object) is equal to other
- returns false otherwise.
- returns true if this (the calling object) is equal to other
int compareTo(String other)
- returns a value < 0 if this is less than other
- returns zero if this is equal to other
- returns a value > 0 if this is greater than other.
- returns a value < 0 if this is less than other
Integer methods and constructors
Integer(value)
- Constructs a new Integer object that represents the specified int value.
Integer.MIN_VALUE
- The minimum value represented by an int or Integer.
Integer.MAX_VALUE
- The maximum value represented by an int or Integer.
int intValue()
- Returns the value of this Integer as an int.
java.lang.Math methods
int abs(int)
- Returns the absolute value of an int value (which means no negatives).
double abs(double)
- Returns the absolute value of a double value.
double pow(double, double)
- Returns the value of the first parameter raised to the power of the second parameter.
double sqrt(double)
- Returns the positive square root of a double value.
double random()
- Returns a double value greater than or equal to 0.0 and less than 1.0 (not including 1.0!)
Common Mistakes: 2.10.4
- Forgetting to declare an object to call a method from outside of the class
- Forgetting parenthesis after method names when calling methods
- Forgetting to give the correct parameters in the right order
- Forgetting to use the value returned from a method
- Using
==
to test the equality of strings- Instead you must use
String.equals
orString.compareTo
- Instead you must use
- Treating upper and lower case letters as the same
- Thinking substrings include the character at the
end
index - thinking strings can change
- they are immutable
- Trying to call a method on a reference variable which hasn’t been initialized yet
- This will cause a null pointer exception
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
This was adapted from the CS Awesome curriculum, which was created by
Barbara Ericson, Beryl Hoffman, and many other CS Awesome contributors. All rights reserved.
CS Awesome is licensed under CC BY-NC-SA 4.0.