Java: byte array elements propageted to int as signed values (0xFF -> -1 instead of 255)

[Mingun]

The problem, similar to #1093, in Java with test tests\formats\expr_bytes_ops.ksy:

meta:
  id: expr_bytes_ops
seq:
  - id: one
    size: 3
instances:
  two:
    value: '[0x41, 0xff, 0x4b]'

  one_size:
    value: one.size
  one_first:
    value: one.first
  one_mid:
    value: one[1]
  one_last:
    value: one.last
  one_min:
    value: one.min
  one_max:
    value: one.max

  two_size:
    value: two.size
  two_first:
    value: two.first
  two_mid:
    value: two[1]
  two_last:
    value: two.last
  two_min:
    value: two.min
  two_max:
    value: two.max

The failure is:

java.lang.AssertionError: expected [255] but found [-1]
	at org.testng.Assert.fail(Assert.java:94)
	at org.testng.Assert.failNotEquals(Assert.java:513)
	at org.testng.Assert.assertEqualsImpl(Assert.java:135)
	at org.testng.Assert.assertEquals(Assert.java:116)
	at org.testng.Assert.assertEquals(Assert.java:389)
	at org.testng.Assert.assertEquals(Assert.java:399)
	at io.kaitai.struct.spec.CommonSpec.assertIntEquals(CommonSpec.java:36)
	at io.kaitai.struct.spec.TestExprBytesOps.testExprBytesOps(TestExprBytesOps.java:17)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:607)
	at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:86)
	at org.testng.internal.Invoker.invokeMethod(Invoker.java:643)
	at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:820)
	at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1128)
	at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:129)
	at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:112)
	at org.testng.TestRunner.privateRun(TestRunner.java:782)
	at org.testng.TestRunner.run(TestRunner.java:632)
	at org.testng.SuiteRunner.runTest(SuiteRunner.java:366)
	at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:361)
	at org.testng.SuiteRunner.privateRun(SuiteRunner.java:319)
	at org.testng.SuiteRunner.run(SuiteRunner.java:268)
	at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
	at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
	at org.testng.TestNG.runSuitesSequentially(TestNG.java:1244)
	at org.testng.TestNG.runSuitesLocally(TestNG.java:1169)
	at org.testng.TestNG.run(TestNG.java:1064)
	at org.testng.TestNG.privateMain(TestNG.java:1385)
	at org.testng.TestNG.main(TestNG.java:1354)

The generated code is:

// Autogenerated from KST: please remove this line if doing any edits by hand!

package io.kaitai.struct.spec;

import io.kaitai.struct.testformats.ExprBytesOps;
import org.testng.annotations.Test;
import static org.testng.Assert.*;
public class TestExprBytesOps extends CommonSpec {

    @Test
    public void testExprBytesOps() throws Exception {
        ExprBytesOps r = ExprBytesOps.fromFile(SRC_DIR + "nav_parent_switch.bin");

        assertIntEquals(r.oneSize(), 3);
        assertIntEquals(r.oneFirst(), 1);
        assertIntEquals(r.oneMid(), 66);
        assertIntEquals(r.oneLast(), 255);
        assertEquals(Long.toString(r.oneLast(), 10), "255");
        assertIntEquals(r.oneMin(), 1);
        assertIntEquals(r.oneMax(), 255);
        assertEquals(Long.toString(r.oneMax(), 10), "255");
        assertIntEquals(r.twoSize(), 3);
        assertIntEquals(r.twoFirst(), 65);
        assertIntEquals(r.twoMid(), 255);
        assertEquals(Long.toString(r.twoMid(), 10), "255");
        assertIntEquals(r.twoLast(), 75);
        assertIntEquals(r.twoMin(), 65);
        assertIntEquals(r.twoMax(), 255);
        assertEquals(Long.toString(r.twoMax(), 10), "255");
    }
}

The type of all value instances here, except *_size, is deduced as Int1Type(false) (i.e. u1) which is rendered in Java as int.

byte in Java is signed, when you assign byte value to an int variable or return it from the int method, the sign is propagated. 0xFF (byte) becomes 0xFFFFFFFF (int) which is -1. To get 255 we need to mask value: byte & 0xFF. There is a question, where this conversion should be applied? Obviously, it should not be applied there:

instances:
  array:
    value: [1, 2, 3]
  array2:
    value: [array.first, array.last, array.min, array.max, array[1]]

I think, we need a dedicated type for "element of byte array" to use in such expressions.