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How Does Java HashMap Work Internally?

HashMap uses a hash algorithm to map keys to array indexes to achieve fast access. 1. First call hashCode() to get the hash value, and then determine the index position through perturbation function processing and modulo acquisition; 2. Use a linked list to resolve hash conflicts. After Java 8, the linked list will be converted into a red and black tree to improve search efficiency; 3. The load factor is 0.75 when expanding capacity, and if the threshold exceeds the rehashing value, it will be rehashing to expand the array capacity; 4. The initial capacity can be preset to reduce the number of expansions to improve performance.

Jul 06, 2025 am 12:22 AM

Purpose and Usage of the 'volatile' Keyword in Java

volatile is a lightweight synchronization mechanism in Java used to ensure variable visibility and orderliness. It is suitable for scenarios where variable changes need to be perceived in a timely manner in a multi-threaded environment. 1. It ensures that the thread reads the latest value of the variable and forces read and write to directly operate the main memory; 2. It prohibits instruction reordering and maintains the program execution order; 3. It does not guarantee atomicity and is not suitable for compound operations such as i. Common uses include status flags, one-time releases, and independent observation updates. When using it, you should cooperate with lock or CAS operations to avoid concurrent errors caused by misuse.

Jul 06, 2025 am 12:03 AM

Explained: Java Polymorphism in Object-Oriented Programming

Polymorphism is one of the core features of Java object-oriented programming. Its core lies in "one interface, multiple implementations". It implements a unified interface to handle the behavior of different objects through inheritance, method rewriting and upward transformation. 1. Polymorphism allows the parent class to refer to subclass objects, and the corresponding methods are called according to the actual object during runtime; 2. The implementation needs to meet the three conditions of inheritance relationship, method rewriting and upward transformation; 3. It is often used to uniformly handle different subclass objects, collection storage and framework design; 4. When used, only the methods defined by the parent class can be called. New methods added to subclasses need to be transformed downward and accessed, and pay attention to type safety.

Jul 05, 2025 am 02:52 AM

java Polymorphism

Implementing and Managing Thread Pools in Java

Java thread pools are created using ThreadPoolExecutor, and you need to pay attention to the core parameter configuration. 1. The number of core threads controls the number of resident threads. It is recommended to set it to the number of CPU cores for calculation-intensive tasks; 2. The maximum number of threads should deal with burst traffic, and excessively high will increase thread switching overhead; 3. The task queue size needs to weigh the risk of memory usage and task loss; 4. The rejection strategy can be customized, such as throwing an exception or executing by the caller; 5. Shutdown should be called first and then combined with awaitTermination and shutdownNow to ensure safe exit. Directly using the Executors tool class can easily cause memory overflow. The submit() method supports obtaining task results. Futur

Jul 05, 2025 am 02:50 AM

java Thread Pool

What is garbage collection in Java and how does it work?

Java's garbage collection (GC) is a mechanism for JVM to automatically manage memory. Its core role is to identify and clear objects that are no longer used to free memory. GC determines whether an object is useless through accessibility analysis. Common GCRoots include local variables in threads, static properties of the class, constant references and JNI references. JVM provides a variety of collectors, such as SerialGC, ParallelGC, CMSGC, G1GC, ZGC and ShenandoahGC, which are suitable for different scenarios. The garbage collection process usually includes three stages: marking, cleaning and sorting. Some collectors such as G1 divide the heap into multiple areas for flexible recycling. Developers can avoid frequent creation of temporary objects, set the heap size reasonably,

Jul 05, 2025 am 02:47 AM

Understanding the Java Virtual Machine Architecture

The JVM architecture consists of three core cores: class loader, runtime data area and execution engine; the class loader is responsible for loading .class files, the runtime data area includes heap, stack, etc. for storing data, and the execution engine is responsible for interpreting or compiling bytecode; the heap stores object instances in the runtime data area, the method area saves class information, and stack management method calls; the class loading mechanism includes three stages: loading, linking, and initialization, and follows the parent delegation model to ensure security; mastering these basic structures helps troubleshoot problems and optimize performance.

Jul 05, 2025 am 02:40 AM

What is Immutability and how to achieve it in Java?

Once an immutable class is created, its state cannot be modified, and any operation will return a new object. To implement immutable classes in Java, the following steps must be followed: 1. Declare the class as final; 2. Set all fields to private and final; 3. Only getter methods are provided, no setters are provided; 4. All fields must be initialized in the constructor; 5. For fields of mutable types, defensive copies are made during assignment and acquisition. In addition, Java 9 supports built-in immutable collections such as List.of(), etc., which helps reduce boilerplate code and prevents accidental modifications. As long as you pay attention to the design of the class and internal state protection, you can achieve true immutability in Java.

Jul 05, 2025 am 02:39 AM

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Understanding Variable Scope and Lifetime in Java

The scope and life cycle of variables in Java depend on type. 1. The scope of local variables is limited to the code block, and the life cycle is destroyed as the code block ends; 2. The scope of member variables is the entire class, and the life cycle is created and destroyed with the object; 3. The scope of static variables is the entire class and can be accessed through the class name, and the life cycle exits from the class loading to the JVM; 4. The scope of parameter variables is limited to the method body, and the life cycle begins and ends with the method call. Variables should be kept as small as possible and short as possible to improve security.

Jul 05, 2025 am 02:36 AM

java variable scope

What is the purpose of the `static` keyword in Java?

Static keywords are used in Java to create variables and methods that belong to the class itself, rather than instances of the class. 1. Static variables are shared by instances of all classes and are suitable for storing data shared by all objects, such as schoolName in the Student class. 2. Static methods belong to classes and do not depend on objects. They are often used in tool functions, such as Math.sqrt(), and can only access other static members. 3. Static code blocks are used to perform initialization operations when class loading, such as loading libraries or setting logs. 4. Static inner classes can be instantiated independently of the external class, but non-static members of the external class cannot be accessed. Rational use of static can effectively manage class-level resources and behaviors.

Jul 05, 2025 am 02:36 AM

java static

Handling Common Java Exceptions Effectively

The key to Java exception handling is to distinguish between checked and unchecked exceptions and use try-catch, finally and logging reasonably. 1. Checked exceptions such as IOException need to be forced to handle, which is suitable for expected external problems; 2. Unchecked exceptions such as NullPointerException are usually caused by program logic errors and are runtime errors; 3. When catching exceptions, they should be specific and clear to avoid general capture of Exception; 4. It is recommended to use try-with-resources to automatically close resources to reduce manual cleaning of code; 5. In exception handling, detailed information should be recorded in combination with log frameworks to facilitate later

Jul 05, 2025 am 02:35 AM

java Exception handling

What is a `CallableStatement`?

CallableStatementinJavaisusedtocallstoredproceduresfromadatabase.1.Itenablesinteractionwithpre-writtenSQLcodeblocksstoredinthedatabasethatcanacceptparametersandreturnresults.2.Ithelpsreducenetworktraffic,improveperformance,andencapsulatebusinesslogic

Jul 05, 2025 am 02:35 AM

jdbc

Using the Java Streams API effectively.

Using JavaStreamAPI can improve code simplicity and parallelism, but the scenarios need to be selected reasonably. 1. Advantages: Declarative programming separates logic and implementation, such as filter, map and collect operations, making filtering, conversion and collection more intuitive; 2. Notes: Avoid modifying external variables in map or filter to prevent concurrency problems; 3. Operation classification: Intermediate operations (filter, map) are lazy to execute, and terminal operations (collect, forEach) are required to trigger execution, and terminal operations cannot be called multiple times; 4. Parallel flow: suitable for large data volumes and complex operations, but performance needs to be tested to avoid shared state operations.

Jul 05, 2025 am 02:23 AM

api

How to use lambda expressions in Java?

Lambda expressions were introduced in Java 8 to simplify the writing of functional interfaces. 1. Its basic syntax is (parameters)->{body}, and the parameter type or brackets can be omitted according to the situation. For example, if there is no parameter, use ()->System.out.println("Hello"), use x->System.out.println(x), use (x,y)->x y for multiple parameters; 2. The lambda can be assigned to functional interfaces such as Runnable, Consumer or Function, instead of anonymous class writing, and improve readability; 3. Often with sets and

Jul 05, 2025 am 02:22 AM

Effective Java Exception Handling Techniques

The key to handling Java exceptions is reasonable response rather than simple capture. First, do not ignore exceptions silently, at least print logs or retow; second, distinguish between recoverable and unrecoverable exceptions. The former uses checkedexception, and the latter uses uncheckedexception; third, avoid excessive use of try-catch, and problems that can be prevented should be checked in advance; finally, properly encapsulate custom exceptions to improve maintainability and context clarity.

Jul 05, 2025 am 02:13 AM