Questions on Java basic plane

1 , what are the aspects of object-oriented features

1. Abstraction: Abstraction is the omission of those aspects of a topic that are not relevant to the current goal in order to pay more attention to the aspects related to the current goal. Abstractions do not intend to understand all of the problems, but simply select one part of them, temporarily without some detail. Abstract includes two aspects, one is the process abstraction, the other is the data abstraction.

2. Inheritance: Inheritance is a hierarchical model of a junction class and allows and encourages the reuse of classes, which provides a way to articulate commonalities. A new class of objects can be derived from existing classes, a process known as class inheritance. The new class inherits the attributes of the original class, which is called the derived class (subclass) of the original class, and the original class is called the base class of the new class (The parent Class). Derived classes can inherit methods and instance variables from their base classes, and classes can modify or add new methods to make them more suitable for special needs.

3. Encapsulation: Encapsulation is to surround the process and data, access to data only through the defined interface. Object-oriented computing begins with this basic concept that the real world can be portrayed as a series of fully autonomous, encapsulated objects that access other objects through a protected interface.

4. Polymorphism: Polymorphism means that objects of different classes are allowed to respond to the same message. Polymorphism consists of parameterized polymorphism and inclusion polymorphism. Polymorphism language has the advantage of flexibility, abstraction, behavior sharing and code sharing, which solves the problem of application function with the same name.

2 , string is the most basic data type ?

Basic data types include byte, int, short, long, char, float, double, boolean

The Java.lang.String class is of the final type, so you cannot inherit the class or modify the class. In order to improve efficiency and save space, we should use the StringBuffer class

3 , int, and Integer, what's the difference?

Java offers two different types: reference types and primitive types (or built-in types). int is the raw data type of Java, and integer is the wrapper class provided by Java for Int. Java provides a wrapper class for each primitive type.

Raw type encapsulation class Booleanboolean Charcharacter bytebyte shortshort intinteger longlong floatfloat doubledouble

The behavior of reference types and primitive types is completely different, and they have different semantics. Reference types and primitive types have different characteristics and usages, including: size and speed issues, which types of data structures are stored as the default values that are specified when reference types and primitive types are used as instance data for a class. The default value of an object reference instance variable is NULL, and the default value of the original type instance variable is related to their type.

4 , String, and StringBuffer differences

The Java platform provides two classes: string and StringBuffer, which can store and manipulate strings, which are character data that contain multiple characters. This string class provides a string of values that cannot be changed. The string provided by this StringBuffer class is modified. You can use StringBuffer when you know that character data is going to change. Typically, you can use Stringbuffers to dynamically construct character data.

5 , final, finally the difference.

Final is used to declare properties, methods, and classes, respectively, that the property is immutable, that the method is not overridden, and that the class is not inheritable. Finally is part of the exception-handling statement structure, which indicates that it is always executed.

6 , overload, and override. Can the overloaded method change the type of the return value?

The overridden overriding and overloaded overloading of a method are different manifestations of Java polymorphism. Overriding overriding is a representation of polymorphism between a parent class and a subclass, and overloading overloading is a representation of polymorphism in a class. If you define a method in a subclass that has the same name and arguments as its parent class, we say that the method is overridden (overriding). When an object of a subclass uses this method, the definition in the subclass is called, and for it the definition in the parent class is "masked". If more than one method with the same name is defined in a class, they either have a different number of arguments or have different parameter types, which is called a method overload (overloading). The overloaded method is to change the type of the return value.

7 , what is the difference between error and exception?

Error indicates a serious problem in situations where recovery is not impossible but difficult. For example, memory overflow. It is impossible to expect the program to handle such situations.

Exception represents a design or implementation issue. That is, it means that if the program runs normally, it never happens.

8 , what is the difference between abstract class and interface? A class that declares the existence of a method and does not implement it is called an abstract class (abstract classes), which is used to create a class that embodies some basic behavior, declares a method for that class, but does not implement the class in that class. An instance of the abstract class cannot be created. However, you can create a variable whose type is an abstract class that points to an instance of a specific subclass. Cannot have abstract constructors or abstract static methods. The subclasses of the abstract class provide implementations for all abstract methods in their parent class, otherwise they are also abstract classes. Instead, implement the method in the subclass. Other classes that know their behavior can implement these methods in the class. An interface (interface) is a variant of an abstract class. In an interface, all methods are abstract. Multiple inheritance can be obtained by implementing such an interface. All the methods in the interface are abstract, without a program body. An interface can only define static final member variables. The implementation of an interface is similar to a subclass, except that the implementation class cannot inherit the behavior from the interface definition. When a class implements a special interface, it defines the method (which is given by the program body) to all such interfaces. It can then invoke the interface's method on any object that implements the interface's class. Because of an abstract class, it allows you to use the interface name as the type of the reference variable. The usual dynamic binder will take effect. A reference can be converted to an interface type or converted from an interface type, and the instanceof operator can be used to determine whether the class of an object implements an interface.

9 , the most common runtime exception.

ArithmeticException, Arraystoreexception, Bufferoverflowexception, Bufferunderflowexception, CannotRedoException, Cannotundoexception, ClassCastException, Cmmexception, Concurrentmodificationexception, DOMException, Emptystackexception, IllegalArgumentException, Illegalmonitorstateexception, Illegalpathstateexception, IllegalStateException, Imagingopexception, Indexoutofboundsexception, MissingResourceException, Negativearraysizeexception, Nosuchelementexception, NullPointerException, Profiledataexception, ProviderException, Rasterformatexception, SecurityException, SystemException, Undeclaredthrowableexception, UnmodifiableSetException, Unsupportedoperationexception

Ten , does the array have the length () method? Does string have the length () method?

The array does not have the length () method, which has the length property. String has the length () method.

Questions on Java basic plane