The maxMemory() method returns the maximum memory that the java virtual machine (the process) can construct from the operating system, in bytes.

The totalMemory() method returns the memory size that the java virtual machine has dug from the operating system, that is, all the memory occupied by the java virtual machine process at that time.

freeMemory() is the process of running java programs. The memory is always slowly dug from the operating system, basically how much is used. But in 100% of the cases of java virtual machine, it will dig a little more. The memory that is dug but not used is actually freeMemory(), so the value of freeMemory() is generally very small.

For example, the following program, in order to more intuitively demonstrate the relationship between the three, I use a for loop to demonstrate:

public void show(View view){
        for(int i=0;i<1000;i++){
            byte[] bytes=new byte[1024*1024*10];//10 megabytes at a time
            long l = Runtime.getRuntime().maxMemory();
            long l1 = Runtime.getRuntime().totalMemory();
            long l2 = Runtime.getRuntime().freeMemory();
            String availMemStr = getSystemAvaialbeMemorySize();
            Log.i("MainActivity", "maxMemory"+formateFileSize(l)+";totalMemory"+formateFileSize(l1)+";freeMmeory"+formateFileSize(l2)+";The Availabel Memory Size is"+availMemStr);
        }
    }

Print the log as follows:

08-08 13:46:17.313 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory12.41 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.316 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory22.44 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.320 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory32.45 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.323 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory32.42 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.326 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory42.44 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.329 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory52.46 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.332 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory62.47 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.335 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory72.48 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.338 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory82.50 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.341 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory92.50 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
08-08 13:46:17.345 8262-8262/green.xi.com.greendaodemo I/MainActivity: maxMemory384 MB;totalMemory103 MB;freeMmeory0 B;The Availabel Memory Size is1.55 GB
.........