ADDITIONAL SYSTEM INFORMATION :
Generic / MacOS / Java 21

A DESCRIPTION OF THE PROBLEM :
**1. MemoryUsage.toString() undefined value display issue**

When `init` or `max` is `-1` (undefined), `MemoryUsage.toString()` outputs `"-1(-1K)"`, which is confusing. While Javadoc states that `-1` means undefined, the output does not clearly indicate this, making it difficult for users to distinguish undefined values from actual negative values.

**2. Constructor error message clarity**

Constructor validation error messages are concise but lack sufficient context for debugging. The current format could be more informative.

**3. toString() performance issue**

The `toString()` method uses string concatenation inside `append()` calls, which prevents proper utilization of StringBuilder's benefits.

### Steps to Reproduce

**Issue 1: toString() with undefined values**
```java
MemoryUsage mu = new MemoryUsage(-1, 1024, 2048, -1);
System.out.println(mu.toString());
// Current output: "init = -1(-1K) used = 1024(1K) committed = 2048(2K) max = -1(-1K)"
// Problem: -1(-1K) is confusing and doesn't clearly indicate "undefined"
```

**Issue 2: Constructor error message**
```java
try {
    new MemoryUsage(1024, 512, 256, 1024); // used > committed
} catch (IllegalArgumentException e) {
    System.out.println(e.getMessage());
    // Current: "used = 512 should be <= committed = 256"
    // Could be more structured and informative
}
```

**Issue 3: toString() performance**
```java
// Current implementation uses string concatenation in append()
// This creates intermediate String objects unnecessarily
MemoryUsage mu = new MemoryUsage(1024, 2048, 4096, 8192);
String result = mu.toString(); // Inefficient string building
```

### Expected Result

**Issue 1:**
```
init = N/A used = 1024(1K) committed = 2048(2K) max = N/A
```
or similar clear indication that the value is undefined.

**Issue 2:**
```
used (512) must be less than or equal to committed (256)
```
More structured and informative error message.

**Issue 3:**
StringBuilder should be used efficiently without intermediate string concatenation.

### Actual Result

**Issue 1:**
```
init = -1(-1K) used = 1024(1K) committed = 2048(2K) max = -1(-1K)
```
The `-1(-1K)` output is confusing and doesn't clearly indicate undefined status.

**Issue 2:**
```
used = 512 should be <= committed = 256
```
Less structured message format.

**Issue 3:**
String concatenation occurs inside `append()` calls, creating unnecessary intermediate objects.

### Proposed Solution

**1. toString() improvements**
- Display `"N/A"` when `init` or `max` is `-1` (undefined)
- Refactor to use StringBuilder correctly
- Maintain existing output format for valid values

**2. Constructor error message improvements**
- Use `String.format()` to provide more structured messages
- Example: `"used (%d) must be less than or equal to committed (%d)"`

**API Compatibility**: Fully backward compatible. Only output format improvements; no impact on existing code.

### Included Change Items

- **#1**: Improve MemoryUsage.toString() handling of undefined values (-1)
- **#4**: Improve MemoryUsage.toString() performance (proper StringBuilder usage)
- **#6**: Improve MemoryUsage constructor error messages

### Test Case Code

**Test for toString() with undefined values:**
```java
/*
 * @test
 * @bug [ISSUE_NUMBER]
 * @summary Test MemoryUsage.toString() with undefined values
 * @run junit ToStringTest
 */

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import java.lang.management.MemoryUsage;

public class ToStringTest {
    @Test
    public void testToStringWithUndefinedInit() {
        MemoryUsage mu = new MemoryUsage(-1, 1024, 2048, 4096);
        String result = mu.toString();
        assertTrue(result.contains("init = N/A"),
                   "Should show N/A for undefined init");
        assertTrue(result.contains("max = 4096"),
                   "Should show max value");
    }
    
    @Test
    public void testToStringWithUndefinedMax() {
        MemoryUsage mu = new MemoryUsage(1024, 2048, 4096, -1);
        String result = mu.toString();
        assertTrue(result.contains("max = N/A"),
                   "Should show N/A for undefined max");
    }
    
    @Test
    public void testToStringWithBothUndefined() {
        MemoryUsage mu = new MemoryUsage(-1, 1024, 2048, -1);
        String result = mu.toString();
        assertTrue(result.contains("init = N/A"),
                   "Should show N/A for undefined init");
        assertTrue(result.contains("max = N/A"),
                   "Should show N/A for undefined max");
    }
}
```

**Test for improved error messages:**
```java
@Test
public void testConstructorErrorMessage() {
    try {
        new MemoryUsage(1024, 512, 256, 1024);
        fail("Should throw IllegalArgumentException");
    } catch (IllegalArgumentException e) {
        String msg = e.getMessage();
        assertTrue(msg.contains("used (512)"),
                   "Error message should include used value");
        assertTrue(msg.contains("committed (256)"),
                   "Error message should include committed value");
        assertTrue(msg.contains("must be less than or equal to"),
                   "Error message should be clear and structured");
    }
}
```

### Test Strategy

**Existing Tests**
- `test/hotspot/jtreg/vmTestbase/nsk/monitoring/MemoryUsage/MemoryUsage/memoryusage001.java` (constructor validation)
- Existing tests need to be updated to match new message format

**New Tests to Add**
- File: `test/jdk/java/lang/management/MemoryUsage/ToStringTest.java`
- Framework: JUnit 5 (`@run junit`)
- Test cases:
  - `init = -1, max = -1` (both undefined)
  - `init = -1, max = valid value` (only init undefined)
  - `init = valid value, max = -1` (only max undefined)
  - Very large values (Long.MAX_VALUE)
  - Zero values
  - `used == committed` (boundary case)

**Test Execution**
```bash
make test TEST="jtreg:test/jdk/java/lang/management/MemoryUsage"
```

### Impact and Risk Assessment

- **Backward Compatibility**: Fully compatible. Only output format improvements
- **Performance Impact**: Minor improvement (StringBuilder usage optimization)
- **Review Difficulty**: Low. Clear scope and testable changes

### First-Time Contributor Suitability

- Clear problem definition and solution approach
- Limited scope and easily testable
- Does not break existing behavior
- Provides good learning opportunity for JUnit test writing
- From reviewer perspective: Output clarity improvements are always welcome