Refuse to write heterogeneous arrays.

src/main/java/com/moandjiezana/toml/BooleanConverter.java

@@ -45,4 +45,9 @@ class BooleanConverter implements ValueConverter, ValueWriter {
   }
 
   private BooleanConverter() {}
+
+  @Override
+  public String toString() {
+    return "boolean";
+  }
 }

src/main/java/com/moandjiezana/toml/DateConverter.java

@@ -119,4 +119,9 @@ class DateConverter implements ValueConverter, ValueWriter {
   }
 
   private DateConverter() {}
+
+  @Override
+  public String toString() {
+    return "datetime";
+  }
 }

src/main/java/com/moandjiezana/toml/MapValueWriter.java

@@ -41,6 +41,7 @@ class MapValueWriter implements ValueWriter {
         valueWriter.write(fromValue, context);
         context.output.append('\n');
       } else if (valueWriter == PRIMITIVE_ARRAY_VALUE_WRITER) {
+        context.setArrayKey(key.toString());
         context.output.append(quoteKey(key)).append(" = ");
         valueWriter.write(fromValue, context);
         context.output.append('\n');

src/main/java/com/moandjiezana/toml/NumberConverter.java

@@ -103,4 +103,8 @@ class NumberConverter implements ValueConverter, ValueWriter {
     return false;
   }
 
+  @Override
+  public String toString() {
+    return "number";
+  }
 }

src/main/java/com/moandjiezana/toml/PrimitiveArrayValueWriter.java

@@ -2,6 +2,8 @@ package com.moandjiezana.toml;
 
 import java.util.Collection;
 
+import static com.moandjiezana.toml.ValueWriters.WRITERS;
+
 class PrimitiveArrayValueWriter extends ArrayValueWriter {
   static final ValueWriter PRIMITIVE_ARRAY_VALUE_WRITER = new PrimitiveArrayValueWriter();
 
@@ -20,12 +22,25 @@ class PrimitiveArrayValueWriter extends ArrayValueWriter {
     }
 
     boolean first = true;
+    ValueWriter firstWriter = null;
+
     for (Object elem : values) {
-      if (!first) {
+      if (first) {
+        firstWriter = WRITERS.findWriterFor(elem);
+        first = false;
+      } else {
+        ValueWriter writer = WRITERS.findWriterFor(elem);
+        if (writer != firstWriter) {
+          throw new IllegalStateException(
+              context.getContextPath() +
+                  ": cannot write a heterogeneous array; first element was of type " + firstWriter +
+                  " but found " + writer
+          );
+        }
         context.output.append(", ");
       }
-      ValueWriters.WRITERS.write(elem, context);
-      first = false;
+
+      WRITERS.write(elem, context);
     }
 
     if (!context.getTomlWriter().wantTerseArrays()) {
@@ -35,4 +50,9 @@ class PrimitiveArrayValueWriter extends ArrayValueWriter {
   }
 
   private PrimitiveArrayValueWriter() {}
+
+  @Override
+  public String toString() {
+    return "primitive-array";
+  }
 }

src/main/java/com/moandjiezana/toml/StringConverter.java

@@ -129,4 +129,9 @@ class StringConverter implements ValueConverter, ValueWriter {
   }
 
   private StringConverter() {}
+
+  @Override
+  public String toString() {
+    return "string";
+  }
 }

src/main/java/com/moandjiezana/toml/TableArrayValueWriter.java

@@ -24,4 +24,9 @@ class TableArrayValueWriter extends ArrayValueWriter {
   }
 
   private TableArrayValueWriter() {}
+
+  @Override
+  public String toString() {
+    return "table-array";
+  }
 }

src/main/java/com/moandjiezana/toml/WriterContext.java

@@ -6,6 +6,7 @@ class WriterContext {
   private String key = "";
   private String currentTableIndent = "";
   private String currentFieldIndent = "";
+  private String arrayKey = null;
   private boolean isArrayOfTable = false;
   private final TomlWriter tomlWriter;
   StringBuilder output = new StringBuilder();
@@ -83,4 +84,13 @@ class WriterContext {
   public TomlWriter getTomlWriter() {
     return tomlWriter;
   }
+
+  public WriterContext setArrayKey(String arrayKey) {
+    this.arrayKey = arrayKey;
+    return this;
+  }
+
+  public String getContextPath() {
+    return key.isEmpty() ? arrayKey : key + "." + arrayKey;
+  }
 }

src/test/java/com/moandjiezana/toml/ValueWriterTest.java

@@ -210,6 +210,18 @@ public class ValueWriterTest {
     assertEquals("", new TomlWriter().write(new TestClass()));
   }
 
+  @Test(expected = IllegalStateException.class)
+  public void should_reject_heterogeneous_arrays() {
+    class BadArray {
+      Object[] array = new Object[2];
+    }
+    BadArray badArray = new BadArray();
+    badArray.array[0] = new Integer(1);
+    badArray.array[1] = "oops";
+
+    new TomlWriter().write(badArray);
+  }
+
   @Test
   public void should_elide_empty_intermediate_tables() {
     class C {