I am from Java background. Although I no longer do Java, I can still introduce the learning method.

It should be noted that the JDK currently supported by Android development is only version 1.6, so you don’t need to pay attention to the new features added in JDK7/8 for the time being.

Phase 1: Basics

At this stage, you need to master the syntax of Java, understand and flexibly use OOP (object-oriented concepts) in Java, master the basic methods of Java program development (command line, IDE, etc.), and complete some tasks on this basis A small program, used as a practice project.

• Understand object-oriented concepts in Java: classes, objects, inheritance, interfaces, overloading, polymorphism, etc.
• Master Java syntax: basic variables, classes and objects, constructors, exception handling, etc. (The content is too fragmentary, here is just a list)
• Learning focus: exception handling, Java data structure, JDBC, GUI, multi-threading. (Summary based on my learning experience)
• Writing programs: You need to write a lot of Java code to deepen your understanding of Java-OOP and Java syntax. It is recommended to try writing small GUI programs, such as Notepad and Minesweeper.
• Recommended book: "Crazy Java Lecture Notes" (for reference only, there are too many such books)

The second stage: Java advanced

At this stage, you need to have a deep understanding of Java's various grammatical rules and design patterns to master how to write relatively high-performance, low-bug, and concise code. This will be based on your extensive Java coding experience.

• In-depth understanding of Java: Master the advanced features of Java.
• Understand design patterns: Design patterns are one of the cornerstones of building large-scale projects in the field of object-oriented programming. Developers can combine different business demand scenarios, select design pattern rules, and build a basic class structure with strong reusability and low coupling. .
• Learning points: These two parts are the key points. Learning them once or twice is not enough. However, except for the first time, subsequent in-depth study will be combined with future project practice, so don’t waste too much time here.
• Writing programs: Can understand the examples in the book and use some of them flexibly.
• Recommended books:
  
  
  • "Java Programming Thoughts" (Fourth Edition)
  • 《Introduction to Design Patterns in a Simple Way》
  • "Design Patterns: The Foundation of Reusable Object-Oriented Software"

The third phase: Android development

I am not a professional Android native application developer, this part is just a brief description.

At this stage, you need to understand the basic methods of Android development, including environment matching, IDE use, writing Helloworld, understanding the Android framework, etc.
There is a lot to learn, but I think I can eat most of it by reading this book.

• Recommended book: "Crazy Android Lecture Notes"

Others

• If you don’t do web development, you may encounter some of Java’s command line tools, such as ant, maven, etc. later, and you can learn them when you encounter them.
• There are many open source Android projects/libraries on Github, you can learn/use them, and you should also learn to use them git.
• When you first start working on an Android App, you will inevitably encounter problems with App design. You can read some books on mobile design.
• As for basic computer content such as data structures, databases, operating systems, and networks, I don’t know your situation. You need to learn based on the actual situation. (There is no harm in learning everything in the computer class.)

As for books, I recommend two books
《Thinking in Java》
《Head First Design Pattern》

Look at the books written by foreign masters. Here is a list of books written by masters, ranging from beginner, intermediate to advanced. 9 must-read books for Java programmers