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Home Java JavaInterview questions Java basic interview questions - generics

Java basic interview questions - generics

Sep 28, 2020 pm 04:26 PM
java Generics Interview questions

Java basic interview questions - generics

139. What are generics in Java? What are the benefits of using generics?

(Recommended more related interview questions: java interview questions And answer )

Generics are a new feature of Java SE 1.5. The essence of generics is a parameterized type, which means that the data type being operated is specified as a parameter.

Benefits:

1. Type safety, providing type detection during compilation

2. Back and forth compatibility

3. Generalized code, the code can be updated More reuse

4. High performance. Code written in GJ (generic JAVA) can bring more type information to the Java compiler and virtual machine. This information can further optimize the Java program. provide conditions.

140, How do Java generics work? What is type erasure? How does it work?

1. Type checking: Provide type checking before generating bytecode

2. Type erasure: All type parameters are replaced with their qualified types, including classes, variables and methods ( Type erasure)

3. If type erasure conflicts with polymorphism, generate a bridge method in the subclass to solve the problem

4. If the return type of a generic method is called is erased, a cast is inserted when the method is called

Type erasure:

All type parameters are replaced with their qualified types:

For example 

T->Object   ? extends BaseClass->BaseClass

How it works:

Generics are implemented through type erasure. The compiler erases all type-related information at compile time, so there is no type-related information at runtime. . For example, List is represented by only one List at runtime. The purpose of this is to ensure compatibility with binary class libraries developed in versions prior to Java 5. You cannot access the type parameters at runtime because the compiler has converted the generic type into a primitive type. Depending on your answer to the generics question, you will be asked follow-up questions about why generics are implemented by type erasure or you will be shown some incorrect generic code that will cause the compiler to error.

141, can you pass List to a method that accepts List parameter?

For anyone who is not familiar with generics, this Java generics question may seem confusing, because at first glance String is a kind of Object, so List should be used when needed List, but this is not the case. Doing so will cause compilation errors. If you think about it further, you will find that it makes sense for Java to do this, because List can store any type of object including String, Integer, etc., while List can only be used to store String s. 

List<Object> objectList;
List<String> stringList;
objectList = stringList; //compilation error incompatible types

142, How to prevent unchecked type warnings in Java?

If you mix generics and primitive types, such as the following code, the javac compiler of java 5 will generate Type unchecked warnings, such as 

List<String> rawList = newArrayList()

Note: Hello.java uses unchecked or unsafe operations;

This kind of warning can be annotated with @SuppressWarnings("unchecked") to shield.

143, What is the difference between List and primitive type List in Java?

The main difference between primitive type and parameterized type is that it is compiled at compile time The compiler will not perform type safety checks on primitive types, but will check types with parameters. By using Object as the type, you can tell the compiler that the method can accept any type of object, such as String or Integer.

The test point of this question lies in the correct understanding of primitive types in generics. The second difference between them is that you can pass any type with parameters to the primitive type List, but you cannot pass List to a method that accepts List, because a compilation error will occur.

144, write a generic program to implement LRU cache?

This is equivalent to an exercise for people who like Java programming. To give you a hint, LinkedHashMap can be used to implement a fixed-size LRU cache. When the LRU cache is full, it will move the oldest key-value pair out of the cache.

LinkedHashMap provides a method called removeEldestEntry(), which will be called by put() and putAll() to delete the oldest key-value pair. Of course, if you've already written a running JUnit test, you're free to write your own implementation code.

(Recommended tutorial: java course)

145, can generics be used in Array?

This may be Java generics This is the simplest interview question. Of course, the premise is that you know that Array does not actually support generics. This is why Joshua Bloch recommends using List instead of Array in the book Effective Java, because List can provide compile-time types. Security is guaranteed, while Array cannot.

146, How to write a generic method so that it can accept generic parameters and return a generic type?

編寫泛型方法并不困難，你需要用泛型類型來替代原始類型，比如使用T, E or K,V等被廣泛認可的類型占位符。最簡單的情況下，一個泛型方法可能會像這樣：

public V put(K key, V value) {
return cahe.put(key,value);
}

147，C++模板和java泛型之間有何不同？

java泛型實現(xiàn)根植于“類型消除”這一概念。當(dāng)源代碼被轉(zhuǎn)換為Java虛擬機字節(jié)碼時，這種技術(shù)會消除參數(shù)化類型。有了Java泛型，我們可以做的事情也并沒有真正改變多少；他只是讓代碼變得漂亮些。鑒于此，Java泛型有時也被稱為“語法糖”。

這和 C++模板截然不同。在 C++中，模板本質(zhì)上就是一套宏指令集，只是換了個名頭，編譯器會針對每種類型創(chuàng)建一份模板代碼的副本。

由于架構(gòu)設(shè)計上的差異，Java泛型和C++模板有很多不同點：

C++模板可以使用int等基本數(shù)據(jù)類型。Java則不行，必須轉(zhuǎn)而使用Integer。

在Java中，可以將模板的參數(shù)類型限定為某種特定類型。

在C++中，類型參數(shù)可以實例化，但java不支持。

在Java中，類型參數(shù)不能用于靜態(tài)方法(?)和變量，因為它們會被不同類型參數(shù)指定的實例共享。在C++，這些類時不同的，因此類型參數(shù)可以用于靜態(tài)方法和靜態(tài)變量。

在Java中，不管類型參數(shù)是什么，所有的實例變量都是同一類型。類型參數(shù)會在運行時被抹去。在C++中，類型參數(shù)不同，實例變量也不同。

相關(guān)推薦：java入門

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