单引用变量如何访问对象所有字段？对象内存地址疑问解析

Let's break down your questions step by step—this is such a core concept that clears up a lot of confusion about how objects work under the hood!

First: What does the memory address in myAnimal look like? #

When you run Animal myAnimal = new Animal();, myAnimal holds a single, fixed-length numeric value (32 bits on 32-bit systems, 64 bits on 64-bit systems; modern JVMs often use compressed pointers to save space). This value is the memory address of the starting position of your Animal object in the heap. Think of it like a street address for the front door of a house—it points directly to where the object begins.

Is it a single address or multiple addresses? #

It's only a single address. Your entire Animal object is stored as a contiguous block of memory in the heap. All its instance fields are stored one after another, starting right at the address stored in myAnimal.

How does a single address let you access all fields? #

Here's the key: the JVM knows exactly how your object is laid out in memory. When your code is compiled (and when the class is loaded by the JVM), it calculates the offset of each field relative to the object's starting address.

For example:

• Suppose your Animal object starts at address 0x12345678.
• The object's header (metadata like type info, lock status, etc.) takes up the first 16 bytes (on a 64-bit JVM with compressed pointers).
• If your Animal has an int age field (4 bytes), its offset from the start is 16 bytes. To access myAnimal.age, the JVM computes 0x12345678 + 16 to find the exact memory location of the age value.
• If there's a String name field next, its offset would be 16 + 4 = 20 bytes, and so on.

These offsets are determined at class load time, so the JVM doesn't have to guess—it just uses simple arithmetic: starting address (from myAnimal) plus pre-calculated offset to locate any field in the object.

内容的提问来源于stack exchange，提问作者Bonsaisteak